GenericValue: drop forced packing pragma
rapidjson.h: explicitly import std::size_t to rapidjson namespace
explicitly qualify C(++) library functions

Some compilers do not export the standard C library functions
to the global namespace, in case the C++ header variants are
included (<cstdlib>, <cstring>).

RapidJSON currently uses:
 * malloc, realloc, free
 * memcpy, memmove, memset, memcpy

Add an explicit namespace qualification to avoid lookup problems.
Merge pull request #148 from pah/fixes/solaris

Alternative compilation fix for Solaris