Add a reverse card feature

[Issue 1555](http://code.google.com/p/ankidroid/issues/detail?id=1555) -- give user ability to add reverse cards.

A lot of people have been complaining about the removal of the swap button, so I decided to go ahead and make a basic feature for generating reverse cards. We only allow the user to add reverse cards when there is a single card template in the note type in question. When this is not the case, the user is given guidance on why the feature doesn't work, and points them to a new [user manual](https://code.google.com/p/ankidroid/wiki/Manual) for v2.3 which I started writing. I believe I've followed the Anki Desktop implementation quite closely.

I had to add a missing method to Collection.java in libanki.

![untitled](https://cloud.githubusercontent.com/assets/2818274/3777413/15a87912-1962-11e4-952a-9589b4b5a892.png)

Any thoughts @dae?