sol::readonly and std.container fixes

This release is a small patchfix to fix the way sol::readonly and std:: containers behave with move-only types. Documentation was updated for sol::readonly and the library was vetted for uses of is_const and is_copy_assignable. Documentation was also updated for Compiler Errors under the Error page in the sol2 documentation: http://sol2.readthedocs.io/en/latest/errors.html#compiler-errors-warnings

Please do upgrade as soon as possible!