Hold-and-move gesture, ViewController-Containment, Cleanup, Empty item spots, Non-centered presentation

Hi!

Thanks again for the great code!
I'm opening this pull request mainly for discussion, I changed quite a lot and I'm not sure if you like all of the changes.

Mainly I added the hold-and-move gesture I asked you about on Twitter, proper ViewController containment and non-centered display (for use in conjunction with a long-press).

Let me know what you think!

cheers, Matthias