issue #280 - refactored DStreamFunctions so that it doesn not call methods of RDDFunctions inside foreachRDD lambdas

DStreamFunctions and RDDFunctions do the same job, but now they do this independently.
Common part of the functionality has been extracted from DStreamFunctions and RDDFunctions into WrittableToCassandra.
WrittableToCassandra trait became an abstract class.
Merge pull request #281 from datastax/wip-280

issue #280 - refactored DStreamFunctions so that it doesn not call metho...