Licenses: prefer spdx.org and improve docs

http://spdx.org was suggested in related threads. It seems the best shot at standardizing license ``shortName``s and providing stable links to the licenses.

I'm not sure about the other style introduced by the third commit: allowing e.g. ``license = licenses."GPL-2.0+";`` in addition to ``license = licenses.gpl2Plus;``.

Any thoughts?