ESC key closes modals

Add a key handler that calls hide() when ESC (keycode 27) is pressed.
Configurable with the escToCancel flag.

@azirbel @ldevalliere 

The _self thing seems funky. I don't know javascript or Ember - teach me! :)
@jcollins1991 pointed out that ```bind()``` here will return a different function than ```bind()``` above, so I made the function a computed instead.