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.

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After going through the documentation for setting up Cassandra as deep storage for druid recently I hit a few road bumps. After looking through the druid-development mailing list, and with help from Fangjin Yang, I was finally able to get Cassandra and Druid working together in harmony.

Relevant mailing list thread: https://groups.google.com/forum/#!topic/druid-development/E1YBo99RcWk