ENH: Add double-conversion library for precise Float output

double-conversion is a library from (or at least hosted by) Google.

https://code.google.com/p/double-conversion/

Double-conversions advantage is that when it converts a floating
point number to a string, it uses the minimum length string representation
that when converted back into a floating point number, the new FP
number is bit-identical to the original.

Change-Id: I308d91a0f8ec6ca2d5d07c0a7b691b44c8fedc7c
Merge topic 'AddDoubleConversion'

8a2205a ENH: Add double-conversion library for precise Float output