[Mailbox-12] Add a Cassandra Mailbox implementation

Hi all ! 

I have worked on an implementation of James-mailbox with Apache Cassandra, based on the Datastax Java driver. Java 7 and a James server with  the Cassandra option in MailboxConfigurationBeanFactoryPostProcessor.java is required.

More information is available in the James-Mailbox Jira: https://issues.apache.org/jira/browse/MAILBOX-12 
Feel free to let any comments, I'll gladly answer them.

Regards,
Philippe