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.

test/unittest/readertest.cpp

@@ -6,8 +6,8 @@
 using namespace rapidjson;
 
 #ifdef __GNUC__
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Weffc++"
+RAPIDJSON_DIAG_PUSH
+RAPIDJSON_DIAG_OFF(effc++)
 #endif
 
 template<bool expect>
@@ -707,5 +707,5 @@ TEST(Reader, Parse_IStreamWrapper_StringStream) {
 }
 
 #ifdef __GNUC__
-#pragma GCC diagnostic pop
+RAPIDJSON_DIAG_POP
 #endif