Release v1.3.1

• Add static assertion to explicitly forbid using non-const lvalue reference
  as argument or using any reference as return for luaw::function.
• Add method luaw::function::unref, luaw::function::ref_id, etc.
• Add new class subluaw to represent a sub thread, and add method luaw::make_subluaw.
• Add method to get main thread.
• Add method luavalueref::unref, luavalueref::setglobal, etc.
• Rename method getvalue of luavalueref to pushvalue.
• Add type check functions is_type_number, is_type_boolean, etc.
• Refactor luaw::to_string to safe version, won't change number to string anymore.
• Refactor push by hint type. Hint could be cv- qualified, and support push
  class instance by a convertible value.