Added mousemove event to allow label to follow cursor

I've added a mousemove event to allow hover labels to follow the cursor. Now, events are only fired when the cursour is entering/leaving an area. The new mousemove event will be fired when the user is moving the cursor within an area, allowing an update to the label position.

![label](https://f.cloud.github.com/assets/548708/123399/3246b7e0-6ebf-11e2-9244-8251c16178b7.png)