Added 'visibility' option; Small clean up in velocity.ui.js

Hey,

thank you for your fast changes!
I tested the "display" skipping, but it doesn't help me :( Look at the example in col 3:
http://codepen.io/anon/pen/BJbtK
(After the block is animated out the opacity will set to 1 -> block is visible)

On the base of this issue https://github.com/julianshapiro/velocity/issues/9 <code> Treat 'visibility' the same way that 'display' is treated </code> i implmented a "visibility" option, like the "display" option.
This work very great:
http://codepen.io/anon/pen/BJbtK

I also tested a "opacity" option. But this makes more problems, because the most animations are base on opacity.

Maybe as idea:
Add an "reset" option, that hold all css modifications which executed after the animation is completed.
This will make big changes to the code, because the "display" option is used very often.