dev-vcs/hub: Add LICENSE (MIT)

Since the fourth day of its existence (2009-12-08), Hub has been distributed
under the MIT license [1,2].

[1]: https://github.com/defunkt/hub/blob/master/LICENSE
[2]:
https://github.com/defunkt/hub/commit/c7e27f43210f5195d9c6fdcb0a8e87246b914abf