Decouple CassandraRow from SparkSQL Row. Fixes #425.

Hmm, actually there is one minor mistake here. Technically CassandraRow is not really internally protected from changing. It is only used as an immutable class and doesn't offer a public API to change its contents. I think it would be little safer to use IndexedSeqs instead of Arrays for data/column names (internally that'd be WrappedArrays).