allow to stream user defined types in a MemoryWriter (#456)

* allow to stream user defined types in a MemoryWriter

* fix indent

* follow Google C++ Style

* make code c++98 compatible

* fix macro usage

* disable ability to stream user defined types if not at least c++11

* fix for disable ability to stream user defined types if not at least c++11

* use FMT_STATIC_ASSERT
3 files changed