Add hover model (indicates current value being hovered)

[rcrisp]

This includes adding a scope variable (hoverModel) that binds the currently "hovered" value. It also switches ng-mouseenter to use ng-mousemove to determine stars being hovered. This allowed me to updated some of the "half star" logic so that half stars are shown in real time while the user is hovering. When I tried the test cases it would only show full stars and then a half star after it was clicked. This does create more scope digests though, as you can imagine. I'm not actually using half stars, so in my case ng-mouseenter would be more preferred. Perhaps in a future update this can automatically be determined and use the appropriate method.

I figured I would include the update for half stars since and leave it at mousemove since I got to tinkering with it anyway. Let me know if there was a reason it was setup the way it was instead. I added an example of the hover model to the index.html example page.