Add more accurate copyright clause

Many people have contributed anad I've made effort to track every one
in README, and their copyrights apply to this work.  So, I don't think
ths change does anything but reflect reality.

LICENSE.md

@@ -1,4 +1,5 @@
-Copyright (C) 2011 Blake Mizerany
+Copyright (c) 2011-2013, 'pq' Contributors
+Portions Copyright (C) 2011 Blake Mizerany
 
 Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: