Updates Cassandra example documentation.

Updates the required properties to enable Cassandra as deep storage.
Also adds more information about how to create the required keystore,
and which properties files need to be modified to support Cassandra.
Also adds syntax highlighting to the provided example code.

Additionally, moves Cassandra Deep Storage documentation out into the
actual documentation hierarchy with appropriate linkage. A stub
README.md is left in the example directory to provide a useful landing
page for existing weblinks to the original documentation.
Merge pull request #680 from lexicalunit/update-cassandra-example

Updates Cassandra example documentation.