Add support for custom user sequences

[jeffmicklos]

PR for changes as that were brought up in Issue #20 (#20).

In summation, this change set adds support for user-defined animation sequences. This has proved to be a huge win, as I am porting all of our animation calls to velocity. We reuse a lot of specific animations and having short-hand access to them is, to put it lightly, life changing. An example of a custom defined sequence would be:

$.velocity.Sequencee.fadeOut = function(options) {
    var opts = { display: "none" };

    var propertiesMap = {
        opacity: 0
    };

    jQuery(this).velocity(propertiesMap, $.extend(options, opts));
};

Now I can just do $('div').velocity('fadeIn'). Awesome. What's even better is we can all imagine a world in which sequences do crazy awesome things beyond just fading in elements.

[jeffmicklos]

Thanks for the super fast action on it. everlane.com should be fully velocity'd by the end of the week!

[julianshapiro]

This is the final comment I will make in this thread:

I've finished documenting Sequences: http://julian.com/research/velocity/#sequences. Thanks again to @jeffmicklos for submitting this genius idea.

[jeffmicklos]

Loved how fast we iterated on this and got it out the door, super excited to see the feature land! If you ever find yourself looking for a job, you know where to find me. :)

[julianshapiro]

<3