Customizable second column for card browser (Issue 1310)

I've made the second column of the card browser semi-customizable, as per [Issue 1310](https://code.google.com/p/ankidroid/issues/detail?id=1310). By default, it shows the answer side of the cards in the second column instead of the deck name, and users can tap on the column2 heading to change  it. It would be nice if we could make this functionality a bit more obvious (an image with a down-pointing arrow maybe?), any ideas on this @agrueneberg?

Rendering of the question/answer is very slow, so I do it in the background after the initial search is complete, and periodically fill in the missing values so that performance isn't affected. The rendering is aborted whenever another background task is initiated, as well as when the user closes the browser activity.

![untitled](https://cloud.githubusercontent.com/assets/2818274/2562087/f5614b00-b838-11e3-87e2-a11857b668ae.png)

I have prepared a binary which people can test out here before the code is merged in the main branch:
https://github.com/timrae/Anki-Android/releases/tag/v2.2browserAlpha3