Updated vnRating directive

@tsanko, @hippee-lee please review the CR.

I have made the following changes:
1. Added ability to set custom CSS classes for filled and empty stars.
2. Changed the `readonly` attribute to `editable` scope property, as it was not working.
3. Consumer can set the maximum number of stars.

Future modifications I'm thinking of are:
1. Remove `translate` dependency and let the consumer set the title (will discuss before doing this).
2. Mechanism to send the updated rating value to the consumer.
3. Add ability to have half / fractional stars (not needed now)