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.

The previous optimization #32 has problem that restoration requires
assignment operator.
Change the backup/restore process using a template wrapper class to
select code path.

https://code.google.com/p/rapidjson/issues/detail?id=75

The modification is slightly different from dlbattle123 to improve
speed.

‘>>’ should be ‘> >’ within a nested template argument list

An unit test is added

git-svn-id: https://rapidjson.googlecode.com/svn/trunk@67 c5894555-1306-4e8d-425f-1f6f381ee07c

git-svn-id: https://rapidjson.googlecode.com/svn/trunk@39 c5894555-1306-4e8d-425f-1f6f381ee07c

git-svn-id: https://rapidjson.googlecode.com/svn/trunk@33 c5894555-1306-4e8d-425f-1f6f381ee07c

git-svn-id: https://rapidjson.googlecode.com/svn/trunk@30 c5894555-1306-4e8d-425f-1f6f381ee07c

git-svn-id: https://rapidjson.googlecode.com/svn/trunk@2 c5894555-1306-4e8d-425f-1f6f381ee07c