cleanup is now working as expected

* If a user posts new feedback, while having already posted one, the value
gets updated. This prevents feedback adding up.
* If feedback is too old (10 minutes), it gets removed. For testing
purposes the cleaning routine currently runs every few seconds, and
feedback only lasts a few seconds as well.
* The deleted feedback changes are broadcasted to:
	o All users, if feedback of other users has been deleted.
	o The user whose feedback got deleted.

Currently, this is a huge mess! There are three types of "Session Ids":
* The Socket.IO "session"
* The internal CouchDB session id
* The ARSnova session keyword

A severe clean up is needed before proceeding any further...

remove websocketcontroller and let sessioncontroller do the
authorization
add toString to User entity

Properties from properties file are available in all beans now.
CouchDB connection in SessionService will be created using these settings.
If a connection fails, the next request will have a new try instead of
throwing an exception.

This method uses transaction level READ_COMMITTED.

A session will be returned if one of the following cases are true:
* the session is active
* the session is inactive AND current (logged in) user is the creator

* Added session controller with method to get session by id
* Created session class to be used as entity object
* Created session service class and interface
* Added content negotiation view resolver to map entity objects to JSON