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.
Merge pull request #5 from willkg/develop

Update license to MPL 2.0