Add MPL 2.0 LICENSE

This PR fixes #1.

I selected the MPL 2.0 license based on the [Mozilla Foundation License Policy](https://www.mozilla.org/MPL/license-policy.html) and because MozStumbler contains source code copied from Fennec.

Does every source file really need the license boilerplate text?

```
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
```