Support for song ratings

I've implemented supported for song ratings, 0 to 10, displayed as 5 stars with 1/2 star resolution.

The code is rather rough as I've written hardly any Java in the last 10 years and I really don't have much of an idea about Android development (frameworks and tooling). Also, I don't know how to keep IntelliJ from trampling over the existing module definitions.

As far as I know, currently only [Cantata](https://code.google.com/p/cantata/) supports ratings. However, as indicated by https://code.google.com/p/cantata/issues/detail?id=555 , may re-introduce them.

Please let me know if you're willing in general to support ratings in MPDroid and in what way I ought to change the relevant code.