update copyright

I blindly copying the license from another repo. Let's put the year the
lib was introduced and include all authors of TOML.

COPYING

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2015 Andrew Gallant
+Copyright (c) 2013 TOML authors
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal