Insert class attribute for card-specific styling

This will support the ability to apply CSS rules to specific cards, in line with the desktop application. So you can now do something like:
```
.card1 {font-size: 20px;}
.card2 {font-size: 40px;}
```
I found three issues in the tracker for it.

[Issue 1246](http://code.google.com/p/ankidroid/issues/detail?id=1246)
[Issue 1568](http://code.google.com/p/ankidroid/issues/detail?id=1568)
[Issue 1530](http://code.google.com/p/ankidroid/issues/detail?id=1530)