* 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