Switch license to Mozilla Public License 2.0

MPL is a copyleft license that is easy to comply with. You must make
the source code for any of your changes available under MPL, but you
can combine the MPL software with proprietary code, as long as you
keep the MPL code in separate files. Version 2.0 is, by default,
compatible with LGPL and GPL version 2 or greater. You can distribute
binaries under a proprietary license, as long as you make the source
available under MPL.