Performance boost a lot!

It is not very useful for iterative parsing as the worst case of heap
size is O(n) where n is number of character in JSON, for the worst
synthetic cases. This is reasonable and should not create stack overflow
security problem as in recursive parsing.

Supplemental documents for iterative parsing

Iterative Parsing (for issue #35)

Improves endian detection

Fix #66 by adding Writer::Reset() and Writer::IsComplete()

Since __BYTE_ORDER__ only avaliable since GCC 4.6.0, add more detection.