C++98 (stlport) support for core FlatBuffers and FlexBuffers.

* Added internal - limited - implementation of flatbuffers::unique_ptr
  for STLs that don't ship with std::unique_ptr.  In C++11 and beyond
  this is just an alias for std::unique_ptr.
* Aliased used type traits structs is_scalar is_floating_point is_unsigned
  into flatbuffers namespace so they can be replaced in C++98 implementations.
  Right now these point at stlport's TR1 implementations.
* Wrapped vector::data() in vector_data().
* Wrapped vector::emplace_back() in vector_emplace_back().
* Wrapper string::back() in string_back().
* Added variants of FlatBufferBuilder::CreateVector() and
  FlatBufferBuilder::CreateVectorOfStructs() that allow the use of plain
  function pointers.
  Generated code has also been modified to use plain functions to build objects
  rather than std::function() so all generated code will work in C++98
  applications.
* Added flexbuffers::Builder::Vector(), flexbuffers::Builder::TypedVector()
  and flexbuffers::Builder::Map() methods that allow the use of plain function
  pointers.
* Changed Parser to internally use plain function pointers when parsing table
  and vector delimiters.
* Added specializations of NumToString() for 64-bit types that aren't supported
  by stringstream in stlport.
* Overloaded numeric_limits for 64-bit types not supported by stlport.
* Replaced build_apk.sh (which was broken by deprecation of the
  "android" tool in the Android SDK) with build.gradle and the
  appropriate gradle wrapper to build an APK.
* Switched Android build to build against all STL variants.
* Updated travis configuration to build Android test and sample.

Tested:
* Verified all tests continue to work on Linux, OSX and Android.
* Verified Travis build is green.

Change-Id: I9e634363793f85b9f141d21454b10686020a2065

include/flatbuffers/idl.h

@@ -20,13 +20,17 @@
 #include <map>
 #include <stack>
 #include <memory>
-#include <functional>
 
+#include "flatbuffers/base.h"
 #include "flatbuffers/flatbuffers.h"
 #include "flatbuffers/hash.h"
 #include "flatbuffers/reflection.h"
 #include "flatbuffers/flexbuffers.h"
 
+#if !defined(FLATBUFFERS_CPP98_STL)
+  #include <functional>
+#endif  // !defined(FLATBUFFERS_CPP98_STL)
+
 // This file defines the data types representing a parsed IDL (Interface
 // Definition Language) / schema file.
 
@@ -164,7 +168,7 @@ template<typename T> class SymbolTable {
   }
 
   bool Add(const std::string &name, T *e) {
-    vec.emplace_back(e);
+    vector_emplace_back(&vec, e);
     auto it = dict.find(name);
     if (it != dict.end()) return true;
     dict[name] = e;
@@ -571,15 +575,32 @@ private:
   FLATBUFFERS_CHECKED_ERROR ParseAnyValue(Value &val, FieldDef *field,
                                           size_t parent_fieldn,
                                           const StructDef *parent_struct_def);
+  #if defined(FLATBUFFERS_CPP98_STL)
+    typedef CheckedError (*ParseTableDelimitersBody)(
+        const std::string &name, size_t &fieldn, const StructDef *struct_def,
+        void *state);
+  #else
+    typedef std::function<CheckedError(const std::string&, size_t&,
+                                       const StructDef*, void*)>
+            ParseTableDelimitersBody;
+  #endif  // defined(FLATBUFFERS_CPP98_STL)
   FLATBUFFERS_CHECKED_ERROR ParseTableDelimiters(size_t &fieldn,
                                                  const StructDef *struct_def,
-              const std::function<CheckedError(const std::string &name)> &body);
+                                                 ParseTableDelimitersBody body,
+                                                 void *state);
   FLATBUFFERS_CHECKED_ERROR ParseTable(const StructDef &struct_def,
                                        std::string *value, uoffset_t *ovalue);
   void SerializeStruct(const StructDef &struct_def, const Value &val);
   void AddVector(bool sortbysize, int count);
-  FLATBUFFERS_CHECKED_ERROR ParseVectorDelimiters(size_t &count,
-                                     const std::function<CheckedError()> &body);
+  #if defined(FLATBUFFERS_CPP98_STL)
+    typedef CheckedError (*ParseVectorDelimitersBody)(size_t &count,
+                                                      void *state);
+  #else
+    typedef std::function<CheckedError(size_t&, void*)>
+            ParseVectorDelimitersBody;
+  #endif  // defined(FLATBUFFERS_CPP98_STL)
+  FLATBUFFERS_CHECKED_ERROR ParseVectorDelimiters(
+      size_t &count, ParseVectorDelimitersBody body, void *state);
   FLATBUFFERS_CHECKED_ERROR ParseVector(const Type &type, uoffset_t *ovalue);
   FLATBUFFERS_CHECKED_ERROR ParseMetaData(SymbolTable<Value> *attributes);
   FLATBUFFERS_CHECKED_ERROR TryTypedValue(int dtoken, bool check, Value &e,