Switched VS build to -W4, and fixed all resulting warnings.

Change-Id: I654217cbd01a3a449503d95753e19b672ec7ec23
Tested: on Windows, Linux

include/flatbuffers/flatbuffers.h

@@ -632,7 +632,7 @@ class Verifier {
     }
     return true;
   }
-
+  
   // Verify this whole buffer, starting with root type T.
   template<typename T> bool VerifyBuffer() const {
     // Call T::Verify, which must be in the generated code for this type.

include/flatbuffers/idl.h

@@ -117,11 +117,11 @@ struct Type {
 
 // Represents a parsed scalar value, it's type, and field offset.
 struct Value {
-  Value() : constant("0"), offset(-1) {}
-
+  Value() : constant("0"), offset(static_cast<voffset_t>(
+                                ~(static_cast<voffset_t>(0U)))) {}
   Type type;
   std::string constant;
-  int offset;
+  voffset_t offset;
 };
 
 // Helper class that retains the original order of a set of identifiers and

include/flatbuffers/util.h

@@ -32,10 +32,16 @@ template<typename T> std::string NumToString(T t) {
   // to_string() prints different numbers of digits for floats depending on
   // platform and isn't available on Android, so we use stringstream
   std::stringstream ss;
-  if (sizeof(T) > 1) ss << t;
-  else ss << static_cast<int>(t);  // Avoid char types used as character data.
+  ss << t;
   return ss.str();
 }
+// Avoid char types used as character data.
+template<> inline std::string NumToString<signed char>(signed char t) {
+  return NumToString(static_cast<int>(t));
+}
+template<> inline std::string NumToString<unsigned char>(unsigned char t) {
+  return NumToString(static_cast<int>(t));
+}
 
 // Convert an integer value to a hexadecimal string.
 // The returned string length is the number of nibbles in