Feature/wait for release

Heya,

This branch is based on current wooga/locker master branch and adds wait_for_release API functions. It's a extension we needed for our locker usage @ SumUp, where we have a thin layer on top of locker which we hope to also either make a pull request out of, or OS as a separate Erlang module. 

For now though we wanted to make a pull request out of this extension until we have tested our other extensions more internally :)