Update license to MPL 2.0

The README says the license is MPL 2.0, but the LICENSE file had the
BSD 3-clause license in it. I checked with Peter and he said it should
be under the MPL 2.0.

This fixes the LICENSE file to contain the MPL 2.0 text.

(I'm lazily doing this from one develop branch to another. If that's problematic, kick me.)

r?