Use Promises

There is a lot of asynchronous code in ShareJS. Almost all methods of the public api use callbacks and many internal methods do, too.

[Promises](http://promisesaplus.com) would be a [good way](http://blog.jcoglan.com/2013/03/30/callbacks-are-imperative-promises-are-functional-nodes-biggest-missed-opportunity/) to deal with this. Of course one should be a able to use the callback-style api if not familiar with promises. But with helpers like [`nodeify()`](https://github.com/then/promise#promisenodeifycallback) we could offer both.