premake.lua: relax warnings for GTest and perftest (fixes #86)

For the "test" solution, the "gtest" and "perftest" projects contain
third-party code.  Relax the warning settings for "gmake" for these
projects to avoid build failures due to third-party code.

Fix parsing numbers which are less than 1e-308

Overflow should check sign of exponent.

Fix SIMD page fault by using aligned load

Related to discussion in #83

Issue75stopwhendone

Add MemoryBuffer and MemoryStream

Iterative parse optimization

Issue31optimizeitoa

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