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! :)
oh you don't need to set _self, you can just pass `@get('keyHandler').bind(@)` to the "on keyup" event so that keyHandler always uses `this` as its context. Then you can change this function to replace all refs to `_self` with `@`.

Also you could just do (replacing _self with @), `@hide() if @get('escToCancel')`. Or, if you have to do this to avoid event bubbling issues, you could just do `return unless @get('escToCancel')`