* unconditionally store error state of reader after parsing
 * clear stack after parsing by using a ClearStackOnExit scope guard

In case of a user-defined RAPIDJSON_PARSE_ERROR_NORETURN that throws
an exception instead of using the Rapidjson ParseError API, the early
return paths performing the stack unwinding manually can be omitted as
well.

This patch provides a customizable RAPIDJSON_PARSE_ERROR_EARLY_RETURN
macro to remove these (then unneeded) control paths from the parsing
implementation (with and without a return value).

Secondly, clearing the parse stack is moved to a small helper struct
that calls stack_.Clear() from its destructor.  This avoids the need
for the 'goto' in the ParseStream function and ensures proper cleanup
even if e.g. a user-defined Allocator throws an exception.

In order to enable the customization of the error macros
 - RAPIDJSON_PARSE_ERROR_NORETURN
 - RAPIDJSON_PARSE_ERROR_EARLY_RETURN
the user may need to have access to the ParseErrorCode enum
already.  This requires a separate header location than the
GenericReader.

While MSVC doesn't like the explicit `.template operator=<...>` syntax
(see 4f40ed64), Clang 3.5 complains about the absence of it:

In file included from ../../test/perftest/rapidjsontest.cpp:6:
../../include/rapidjson/document.h:504:18: error: use 'template' keyword to treat 'operator =' as a dependent template name
                return (*this).operator=<StringRefType>(str);
                               ^
                               template

Delegate both operator=(StringRefType) and operator=(T) to operator(GenericValue&).

Fixes and cleanups for MSVC 

There's no need to enforce this flag during the build of the perftest
or the GoogleTest library, as both include third-party code.

There are two copies of `StrLen` in the RapidJSON code base
 * strfunc.h: rapidjson::internal::StrLen<Ch>
 * unittest.h: Strlen<Ch>

To hide a warning on MSVC, align both implementations to use
'unsigned/SizeType' as return type and add an explicit cast.

The `StringRefType` assignment operator overload
leads to a compiler error on MSVC 2005 and later:

..\..\include\rapidjson/document.h(504) : error C2951: template declarations are only permitted at global, namespace, or class scope

Drop the unneeded 'template' keyword here.

MSVC with enabled RAPIDJSON_SSE2/RAPIDJSON_SSE42 requires the explicit
definition of the `_BitScanForward` intrinsic.  This can be reliably
ensured by including "intrin.h" and properly marking '_BitScanForward'
as intrinsic.

Confirmed on MSVC 2005, 2008.
Should fix https://code.google.com/p/rapidjson/issues/detail?id=96

travis-doxygen.sh: avoid leaking the GitHub token in build log

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.