Warning push/pop support has been added to GCC in version 4.6.0,
and pragmas to ignore certain warnings are present since 4.2.0.
This patch hides the compiler-specific warning push/pop/disable
pragmas behind a macro-based implementation (currently for MSVC and
clang /GCC.

This avoids warnings, as seen e.g. on GCC 4.4:
  ../../include/rapidjson/document.h:14: error: expected [error|warning|ignored] after ‘#pragma GCC diagnostic’
and earlier versions complaining about unknown pragmas being ignored.

Note: unittest.h and perftest.h need to check for compilers
explicitly, as rapidjson.h is not included there.

Some links will become broken in GitHub.

Travis doxygen

Imitate readthedocs

Imitate GitHub markdown rendering

Fixes diagrams URL in DOM

Conflicts:
	doc/stream.md
	doc/tutorial.md

But some examples are to be filled in.

Integrate Markdown pages into Doxygen documentation

To avoid the {#mainpage} tag to be rendered on GitHub, explicitly add a
`\mainpage` section to `rapidjson.h`, referring to `readme.md`.