Improved handling of (constant) strings

Fixup travis doxygen build script

Travis CI: build/update Doxygen from separate script

With the new string handling API, the constructor taking a `bool`
parameter matches in some unwanted cases, as pointers can be casted
to `bool` implicitly.

Add a SFINAE helper to this constructor to avoid matching arbitrary
pointers.  To avoid confusion for the user, this mechanism is hidden
from the Doxygen documentation.

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.

To reduce repetition and to provide a proper type hierarchy,
let type traits inherit from TrueType and FalseType.

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