Restapi/event

based on:
https://sat.inso.tuwien.ac.at/redmine/projects/qse-ase-ws14-11/wiki/Rest_API_Description

Not implented from there:
DELETE /event - deletes an event (useful for debugging - not sure if users have access to this)
=> use Mongo DB for this
PUT /event - updates the event information
=> we deceided in a meeting that we do not change the settings during an event, therefor no put

Additionally implemented:
GET /event/active - lists all active events
PUT /event/current/end - ends the current event