More native code gen functionality.

Allow tables to be mapped to native types directly.  For example, a table
representing a vector3 (eg. table Vec3 { x:float; y:float; z:float; }) can
be mapped to a "mathfu::vec3" native type in NativeTables.  This requires
users to provide Pack and UnPack functions that convert between the
Table and native types.  This is done by adding the "native_type" attribute
to the table definition.

To support user-defined flatbuffers::Pack and flatbuffers::UnPack functions,
support a "native_include" markup that will generate a corresponding

Also add an UnPackTo function which allows users to pass in a pointer to
a NativeTable object into which to UnPack the Table.  The existing UnPack
function is now simply:

  NativeTable* UnPack() {
    NativeTable* obj = new NativeTable();
    Table::UnPackTo(obj);
    return obj;
  }

Finally, allow native types to be given a default value as well which are
set in the NativeTable constructor.  This is done by providing a
"native_default" attribute to the member of a table.

Change-Id: Ic45cb48b0e6d7cfa5734b24819e54aa96d847cfd

docs/source/Schemas.md

@@ -309,6 +309,10 @@ Current understood attributes:
     to be stored in any particular order, they are often optimized for
     space by sorting them to size. This attribute stops that from happening.
     There should generally not be any reason to use this flag.
+-   'native_*'.  Several attributes have been added to support the [C++ object
+    Based API](@ref flatbuffers_cpp_object_based_api).  All such attributes
+    are prefixed with the term "native_".
+
 
 ## JSON Parsing