simple_usertype, not so simple anymore

This release heavily improves simple_usertype to make it equivalent and equal to the usertype. It introduces a breaking change in that variables are no longer serialized as functions: to do that, one must wrap any bindings in sol::as_function.

The caveats and problems are no longer present from old versions, and the documentation has been updated: http://sol2.readthedocs.io/en/latest/api/simple_usertype.html

sol::as_function, as mentioned before, will now turn any "my_variable", &my_type::my_variable binding entry into a callable function.

Have fun!