Unbinds an event listener previously bound with up.on().
Example
Let's say you are listing to clicks on .button elements:
var listener = function() { ... };
up.on('click', '.button', listener);
You can stop listening to these events like this:
up.off('click', '.button', listener);
Note that you need to pass up.off() a reference to the same listener function
that was passed to up.on() earlier.
