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.
Should return DataContext (the interface) and not QueryPostprocessDataContext (an abstract class which we don't want to expose to end-users).