With the allowance of arbitrary root value types, the individual tests
can use the top-level Parse functions, instead of ParseFoo() variants.

Secondly, some unneeded array wrappers have been dropped and non-singular
tests starting with other values than objects or arrays have been added.

Overflow should check sign of exponent.

Related to discussion in #83

It is not very useful for iterative parsing as the worst case of heap
size is O(n) where n is number of character in JSON, for the worst
synthetic cases. This is reasonable and should not create stack overflow
security problem as in recursive parsing.

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