Previously, a single wrong answer was sufficient for a
question to get a 0% score. With this change, they
are counted as much as a correct answer. The only
difference now is that they reduce the overall
score of a question. See test cases for a
few examples.

The numbers for 'sessions' and 'questions' are now
derived from their underlying elements:
- open/closed for sessions
- lecture/preparation for questions

Removed test cases because they weren't of much use after
the refactoring.

This will result in events being fired at the right time.
Most importantly, this ensures event-specific caching
annotations to work correctly.

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.