Change license from LGPL to Mozilla Public License

I technically wasn't complying with LGPL in the first place, since I
didn't include a complete copy of the license; MPL just requires I link
to it, as I do now.

Besides the pretty website, MPL is just shorter, simpler, and easier to
understand.
Merge pull request #283 from mathquill/docs.license

Change license from LGPL to Mozilla Public License