Improves the default KVStore implementation in NextSongRecommender.

The change to the default behavior of KijiURI builders allows us to define a useful default implementation of the KVstore to use in our recommendations producer. I think the KijiURI Exceptions are also unchecked now, which means we can remove the ugly try/catch. Unfortunately, maven is flailing around with fake errors for me locally:
 
[ERROR] /home/juliet/src/kiji/kiji-music/src/main/java/org/kiji/examples/music/gather/SequentialPlayCounter.java:[84,12] cannot find symbol
[ERROR] symbol  : method newColumnsDef()
[ERROR] location: class org.kiji.schema.KijiDataRequestBuilder

Instead of fighting with maven (for more than an hour) over something I know should work, I'm moving on, and hoping you can build this correctly.