Changing the Session object the way it was done previously
interferes with the new caching mechanism. The objects are
now long-lived so any direct changes to it which are not
in the spirit of its use case have to be removed. In this
case, changing the creator information invalidated many
of the isCreator checks sprinkled through the code, as
the new creator's name would have been the string
"not visible to you."

Updated copyright year and adjusted header format to be consistent over
all files and with other ARSnova projects.

CouchDB tries to set _conflicts propertiy but did not find a setter and
throws NoSuchMethodException. This patch will avoid exception throwing.

The reason of exception throwing can be found can be found in CouchDBs
availibility to store different and concurrent versions of one single
document. This error occurs if two different versions of one session
document are stored within CouchDB.

Further we have to take care, that there is only one version of a
session, not multiple versions.

* 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