refactor($ionicPopup): add close() to returned promise, change closing

BREAKING CHANGE: $ionicPopup.show()'s button onTap function has changed.

When using `$ionicPopup.show()`, previously a button's onTap function
would only result in closing the popup and resolving the promise if the
 `onTap(event)` function returned a truthy value.

Now, a button's onTap event will *always* close the popup and resolve
the popup's promise, no matter the return value, by default. The only
way to prevent the popup from closing is to call
`event.preventDefault()`.

Change your code from this:

```js
$ionicPopup.show({
  buttons: [{
    onTap: function(event) {
      if (!shouldClosePopup) {
        return false;
      }
    }
  }]
});
```

To this:

```js
$ionicPopup.show({
  buttons: [{
    onTap: function(event) {
      if (!shouldClosePopup) {
        event.preventDefault();
      }
    }
  }]
});
```