`display` skipping & one-time callback execution

You can now skip having the UI Pack effect change animating elements' `display` property by passing in `display: null` as an option into the sequence call.

Further, callback behavior now mimics that of standard Velocity animations: `begin` and `complete` are only fired once, regardless of how many elements were passed into the Velocity call. (Prior to now, they were firing once per element in the call.)

Thank you to @devertnet and @bendiksolheim.
Merge branch 'master' of https://github.com/julianshapiro/velocity into julianshapiro-master

Conflicts:
	velocity.ui.js