Change double parsing with fast-path conversion

Accurate rounding in normal numerical ranges, also reduce lookup table
size.
Custom dtoa() impleemntation

Modified from Milo's Grisu2 implementation. 99.9% cases return shortest
decimal format.
Fixed gcc effc++ warning in dtoa.h
Remove double precision settings API in Writer
Change CountDecimalDigit32() to simple implementation

It is simple and pure C++. And it is found in performance test that it
is even faster than the original version, due to distribution of n. But
the performance gain is not obvious in RapidJSON.
Merge pull request #97 from miloyip/issue72customdtoa

Fix #72