feat(carousel): Callbacks when carousel transitions begin and end.

This commit adds callbacks for the begin and end of the transition animations.
When a transition starts, the begin callbacks are invoked (for entering or leaving slides).
When the transitioning ends, the end callbacks are invoked (for entering or leaving slides).

This introduces the following attributes on the slide directive:
* onenteringtransitionbegin
* onenteringtransitionend
* onleavingtransitionbegin
* onleavingtransitionend

These will cause the corresponding callback expression to be evaluated (within the scope).

Example:

<pre>
&lt;slide ... onenteringtransitionbegin="enteringEnded()" ...>
</pre>
Where the scope has the following function:  
<pre>
$scope.enteringEnded = function enteringEnded() { ... };
</pre>