Visibility Toggling

You can now set the `visibility` option just like you can `display`.
See http://velocityjs.org/#displayAndVisiblity. Closes #9.

Fixed bug that occurs with stagger + out transitions (from the UI
pack). Closes #164. Thank you, @codedependant.

You can now pass a function to the `stagger` option. Closes #163.
Closes #162. Thank you, @codedependant and @cihadturhan.