Travis CI: build/update Doxygen from separate script

In this patch, a script 'build/travis-doxygen.sh' is added to
build and push the Doxygen documentation to the GitHub pages at
https://miloyip.githib.io/rapidjson.

The script exits gracefully, if the build is requested for
 - a branch other than 'master'
 - a pull-request
 - a job (i.e. CI configuration) other than "1"

In case the "secure variables" are not available, only the final upload
is skipped, in order to allow testing of the script's basic functionality.

Update .travis.yml to call the script.

Add RAPIDJSON_DIAG_* macros to handle compiler warnings

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