Mention another third-party piece of code we use
Add MPL copyright header to all of our files

As per https://www.mozilla.org/MPL/headers/
Add a full copy of the MPL to the repo
Remove the license section from the README

Now that there is a copy of the MPL in the LICENSE file, this
section is redundant.

Also, the tri-license stuff is no longer needed with version 2
of the MPL.
Merge pull request #569 from fmarier/add_licensing_headers

Add licensing headers