Added "tap and hold gesture" to move cells.

Finally, I've implemented the "tap and hold gesture" to move cells around. IMHO, it's a really well implemented feature.

One little detail is that when the cell is at the bottom or top of the table view it does not scroll automatically to continue moving it. The reason is, basically, that I don't really know how to implement that function without creating an infinite loop and crashing the app (:P) so some help would be really appreciated.