This also fixes a test error.

Caching was too aggressive. However, the database
has been asked for session info even if there
were no actual sessions! A simple check now saves
four immediate CouchDB requests.

getSession throws an HTTP exception, which is not
good style. Additionally, as it calls
getSessionFromKeyword internally -- which has a caching
annotation -- this caching mechanism will not work
because it only works if the method is called
from another object. This is due to the way Spring
is using proxies to intercept the calls.

Events for all learning progress related actions
are generated, like answering a new question.
The event classes are used as a tag to evict
the cache based on the session.

There might be answers to questions without values,
those would have hit the exception. So it is better
to just ignore those questions.

Conflicts:
	src/main/java/de/thm/arsnova/controller/SessionController.java
	src/main/java/de/thm/arsnova/dao/CouchDBDao.java
	src/main/java/de/thm/arsnova/dao/IDatabaseDao.java
	src/main/java/de/thm/arsnova/services/ISessionService.java
	src/test/java/de/thm/arsnova/dao/StubDatabaseDao.java

Conflicts:
	pom.xml
	src/main/java/de/thm/arsnova/controller/SessionController.java
	src/main/resources/log4j-dev.properties