New welcome screen

The tutorial deck generation is broken in 2.3 (see [issue 2306](https://code.google.com/p/ankidroid/issues/detail?id=2306)), and rather than fixing it, I thought it was better to just fast-track the [changes I've been planning](https://groups.google.com/d/topic/anki-android/cZ6gtU3vh_M/discussion) which would replace the tutorial deck.

This PR uses the [ShowcaseView](https://github.com/amlcurran/ShowcaseView) library to show a welcome screen overlay whenever the DeckPicker is opened with an empty collection. The overlay points the user to the + button in the deck picker for adding content, and also to the getting started section of the user manual which goes into more detail. I will also add some tutorial videos there in the near future so the tutorial deck is no longer needed.

![device-2014-10-06-002747](https://cloud.githubusercontent.com/assets/2818274/4518942/1c2c73e2-4ca7-11e4-8a10-bbedfe69141f.png)

The text position and radius of the circle could do with a bit of tweaking, but I think this is fine for now (the text only overlaps like this on tiny QVGA screen), and I'd like to push this into 2.3.