Standardize copyright, add root-level CONTRIBUTORS credit

This assigns copyright universally to BVLC and contributors and credits in
a top-level CONTRIBUTORS.md with a link to the github contributors page, as
discussed in #249.

To further give credit where credit is due, instructions are included on seeing
contributions line-by-line (`git blame`) and through history
(`git log --follow`).

Please merge once all the votes are in at #249.