Cassandra DB support for MetaModel [METAMODEL-18]

Cassandra DB support using the datastax java driver, see: https://github.com/datastax/java-driver

Integration tests have been also added. These tests need a cassandra instance running, properties to enable the integration test for this module have been added to the example-metamodel-integrationtest.configuration.properties file.
I wonder if we need to close something ... Obviously the Iterator is not closeable, but maybe we forgot to carry along the thing that is closeable? That would be quite common when dealing with a streaming resource.