Added MinGW threading Capability

[ghost]

I added this for the users who want to use MinGW without boost, i added meganz/mingw-std-threads as a submodule, and updated thread.hpp to include the mingw headers, if required

[zaphoyd]

I've merged this patch with a few changes:

• I don't want to bring in new hard dependencies so I've removed the submodule to mingw-std-threads. Whether to include this code is something the library user should decide.
• Since the referenced git repo is one of multiple solutions to threading on MinGW I don't want to hard code the decision to always use it for all library users. A MinGW user could want to use Boost Thread for example instead. As such I've removed the automatic inclusion of these headers if MinGW is detected.
• Rearranged the checks for mingw-threads to be first/highest priority in the list so that if the user defines _WEBSOCKETPP_MINGW_THREAD_ then WebSocket++ will definitely use that library instead of the STL or Boost.
• Updated the documentation & FAQ to point MinGW users towards this library as an alternative to Boost threads.