Modify cassandra.* config setting for use with spark-shell

Modify cassandra.* configuration settings to prefix with `spark`. so they can be used from `spark-shell` and set via `conf/spark-defaults.conf`.  Refer to issue #44 for more information.

The default configuration options are not recognized by Spark so if attempting to connect to a remote host with the spark-shell, there was no way to achieve this.  By adding `spark.` to the configuration options, they can now be set in the `conf/spark-default.conf` file and appended to the SparkContext for reference by the spark-cassandra-connector.

-Todd