Allow confirm on enter in modal dialog

Confirm modal dialog on enter, and make sure cancel/confirm callbacks are actually called on keystrokes. Depends on/has conflicts with https://github.com/Addepar/ember-widgets/pull/107

@azirbel @Dylnuge 
I know this wasn't you initially but I think comments are better on their own lines, i.e.

```
# On Escape keypress
if event...
# On Enter keypress
else if event...
```