Cassandra Plugin

This pull request contain an updated version of Martin Weindel's Cassandra plugin for Presto.  The changes include the following:

Updated the code to the Presto coding style. (https://github.com/airlift/codestyle)

Split CassandraClient into separate services.  This is how the other connectors are designed, and Hive is only this way for legacy reasons.

Remove the ClassLoader wrappers because Cassandra client does not seem to load class dynamically (like Hive does).  We can add this back if we find out the client is loading classes.

Added some initial tests that start an embedded Cassandra server.  We need much more extensive tests, but I think this is a good starting point.