feat(rating): add onHover and onLeave.

Thought it would be useful to let users specify custom on/off classes (defaults to ' icon-star' and 'icon-star-empty'). For example: various bootstrap icons, dollar signs, on/off indicators, etc.

Also added { onHover: '&' } to the scope. This fires every time scope.val changes (via $watch), giving the parent scope optional visibility into the icon states. This is useful if you want to show a rating description, a complimentary image or other mark-up while a user hovers over a particular rating.

Note: both of these features are entirely optional.