Parser now correctly reads floats in scientific notation.

Change-Id: I7abb14a4b6c596674d6aff2b9de6e63603c0d2dc
Tested: on Windows and Linux.
Changed the tracking token implementation to be more robust.

The previous version only was included for users of FlatBufferBuilder,
this one should also work for programs that only ever read
FlatBuffers.

Bug: 15762139
Change-Id: Ifceb337ffc1dd9dd09d77c3848cad8707e5c3726
Tested: on Linux.