Fix timeboxing and clean up old session code

These changes update the timeboxing behaviour so it works like on the desktop client and fixes [Issue 1355](http://code.google.com/p/ankidroid/issues/detail?id=1355).

Since sessions were removed in Anki 2 and timeboxing was closely related to that feature in the old version, I've taken the opportunity to remove anything related to sessions as well. I've left some of the ```studyoptions_*``` strings since I think they might be of use as filtering selections later on.

One thing to note about this commit: 
The rep count is incremented whenever the next card is fetched, which can be seen in DeckTask.doInBackgroundAnswerCard(). That's good so far. However, AnkiDroid appears to answer a "dummy" card when it first starts a review session (see the end of Reviewer.onCreate()). This prematurely increments the rep count, resulting in the first timebox being 1 more than it actually is and having one less rep for the final rep.

In both cases, I've manually accounted for the extra and missing reps at the start and end. It doesn't feel like the best solution, but it's the simplest I could come up with.