Fixes and cleanups for MSVC 

This pull-request adds some fixes and cleanups for MSVC.
Tested on MSVC 2005 (32/64), 2008 (32), and 2013 (32/64).

The [issue regarding SSE2 on MSVC'2003](https://code.google.com/p/rapidjson/issues/detail?id=96) should be fixed as well.