CassandraConnector shares core driver Session objects. Previously it shared only Cluster objects and a separate Session was open per every Spark task. This had a significant performance overhead in terms of CPU, networking and memory use, because every Session object manages an internal pool of connections to all cluster nodes. Fixes #41 and #53.
Improved JavaDoc explaining CassandraConnector withClusterDo and withSessionDo semantics.
Merge remote-tracking branch 'origin/master' into issue-41

Conflicts:
	src/main/scala/com/datastax/spark/connector/cql/SessionProxy.scala
Merge pull request #54 from datastax/issue-41

CassandraConnector shares core driver Session objects. Previously it sha...