Problem: using custom LGPLv3+ license

The ZeroMQ license (LGPLv3 + static link exception) works well except
for being a custom license that causes issues with some legal teams.
To all extents the MPLv2 license now offers the same model of sharing
patches while allowing static linking in any application.

This patch applies the MPLv2 to this project.
Merge pull request #2 from hintjens/master

Problem: using custom LGPLv3+ license