Many improvements

[CocoaBob]

Hi,

I've made many improvements to this project. First of all, I've imported most of the useful changes from other contributors. Then I've added 3 main changes:

1. Keep tracking dragging when moving up, instead of doing nothing, so that it could feel nature.
2. Added customizable velocity and distance to dismiss the modal view more flexibly.
3. Added a delegate to check if we should begin the pan gesture, to avoid gesture conflicts.

Thanks!