to_args and thread changes

This minor release adds a new construct for argument passing from C++ to Lua and also adds some minor changes to thread to make it easier to work with.

• New as_args, as basically requested by @eevee: http://sol2.readthedocs.io/en/latest/api/as_args.html
  • Note that the implementation of this was piss-easy to do as a customization point: if you need a feature, you can probably implement it as a customization point
• Documentation updates
• New constructor and member function for thread: http://sol2.readthedocs.io/en/latest/api/thread.html