Support for saving RDD[CassandraRow]. New demo program copying data from...

... one table to another. Fixes #16.
It would be nice if the `selectedColumns` contains a set of columns used to intersect the columns present in a particular row rather than a set of mandatory columns. Rows may differ and the writer may omit missing columns (given that they are not included in the primary key). IMHO it would be more flexible.