add custom study button to study options fragment

As per [issue 1569](https://code.google.com/p/ankidroid/issues/detail?id=1569), Damien requested a custom study button which was targeted for 2.1. The custom study dialog and graphical tag selector were [already implemented by Pablo](https://github.com/ankidroid/Anki-Android/commit/a7c98eb1e055a824d84710edba71d17ac6038687), and I thought we could keep it for 2.2, but **I think it's a bit of a waste to not have a button which takes advantage of Pablo's changes in 2.1.**

The new layout looks like below; I thought it looked strange just having the custom study button alone, so I made it look the same as desktop (which included changing "start reviewing" to "study now")

![new_layout](https://f.cloud.github.com/assets/2818274/2460848/7bfcf712-af70-11e3-8999-289c37616853.png)