Implemented a patch method which merges an existing entity with updated
properties from a HashMap. A new test dependency spring-security-test
has been added.

* Use consistent permission for all target domain objects:
    * read, create, owner, update, delete
* Use switch blocks to improve readability
* Fetch target domain objects early to reduce code duplication

Repositories now implement Spring Data interfaces. Basic CouchDB
operations are now implemented once by `CouchDbCrudRepository` instead
of having redundant methods to handle each design document.

* Prefer ids over "foreign" domain objects.
* Consistently use final keyword for implementation parameters and
  variables.
* As a consequence, session id is used instead of the object as cache
  key for `skillquestions`, `lecturequestions`, `preparationquestions`
  and`flashcardquestions` caches.
* Rename occurances of `questionId` to `contentId`.

Added new service for comments.

* Moved classes from `de.thm.arsnova` root package.
* New util package
* Do not start interface names with `I`. Appended `Impl` to the
  implementing class instead.
* Renamed learning progress classes to score and move them from
  `.domain` to a new `.services.score` package.
* Do not capitalize acronyms in class names:
    * CASLogoutSuccessHandler -> CasLogoutSuccessHandler
    * SocketIO* -> Socketio*
* Renamed `.socket` package to `.websocket`

Fixes a regression introduced by commit
2ecbfec8.

Authentication is needed if an CouchDB admin user exists for design doc
creation at startup.

Closes GH-6.

Mosts of the tests were coupled with the old database layer.
Additionally, a lot of them were testing simple CRUD operations intead
of domain logic. Meaningful tests with proper database mocking have to
be written at a later time.

Code for the following domains has been migrated:
* VisitedSession
* MotdList
* Statistics
* LearingProgress

(Lecturer/Skill)Question has been renamed to Content. Method names have
not been touched yet to ease reviewing the changes.

InterposedQuestion has been renamed to Comment. Method names have not
been touched yet to ease reviewing the changes.

LoggedIn documents are also handled by the SessionRepository for now.

Java's ScriptEnging is used to transform design doc JS objects into
valid JSON. A custom CouchDbConnector has been implemented to handle
this transformation and create the design docs in the database at
startup.

Priortizing v3 caused problems for v2 clients on POST requests.

This decouples type identifiers used in CouchDB from Java type names.

The creation of new documents is blocked if an empty _rev is set.

Ektorp is introduced as a replacement for CouchDB4J.

Functions need to be stringified to create valid JSON which is accepted
by CouchDB.