Fixes#491 (and probably #480) Before, the put-area of the custom streambuf
implementation was (sometimes) incorrectly extended beyond the writeable buffer.
The new implementation is in some cases not as efficient as the old, but avoids
to write into uninitialized memory.
* 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
