Document traits of types using <type_traits> and static_assert in tests

These additional tests state the current traits of `Document`, `Value` and `StringBuffer`. There are slight differences between them. It seems like a good idea to extend this approach across more types, and to review the expected traits across the board.

For now, this piggybacks on `RAPIDJSON_HAS_CXX11_RVALUE_REFS` to determine whether the compiler is C++11.