Merge pull request #2 from martabel/new-engage-player

MH backbone example
Engage: plugin api, plugin manager, example plugin
Merge pull request #3 from bwulff/new-engage-player

Engage: plugin api, plugin manager, example plugin
Merge branch 'new-engage-player' of https://github.com/CallToPower/Opencast-Matterhorn into new-engage-player-testing

* 'new-engage-player' of https://github.com/CallToPower/Opencast-Matterhorn:
  Engage: plugin api, plugin manager, example plugin