Update copyright model

This is a replacement for @pelson's #1818, and updates the copyright model.

Previously, John Hunter held the sole copyright for all matplotlib code.  As he, very unfortunately, is not longer with us, we do need to address that.

Heavily influenced by the IPython licensing, I propose to move us to a shared copyright model, where authors retain copyright on their individual contributions, but the code base as a whole belongs to the entire community of contributors.  This does not actually change the license from the BSD one that we have had all along, so should have no impact on its usability in or with other projects.