Add Mozilla Public License v2.0

FAQ about the MPL: https://www.mozilla.org/MPL/2.0/FAQ.html

Short explanation: the copyleft applies to any files containing MPLed code. You are free to use the software for your private usage and redistribute the software without modifications. You are free to modify the software but you should always provide the source files of your modifications and release them with the MPL. If you use the software (modified or not) in any larger project, including proprietary software, just the originally MPL sources have to be redistributed as that, if modified.