fix($ionicActionSheet): fix problems with cancel() not being called

Closes #1013, #1576.
fix($ionicActionSheet): stop memory leak due to hidden element staying in dom

BREAKING CHANGE: $ionicActionSheet now returns a method to hide the
action sheet.

Previously, it returned an object that had a `show` and `hide` method.
This was undocumented, but if you used it, here is how to migrate your
code:

Change your code from this:

```js
var sheet = $ionicActionSheet({...});
sheet.hide();
```

To this:

```js
var hideSheet = $ionicActionSheet({...});
hideSheet();
```