Updated titan-cassandra dependencies

Added JNA as an optional dependency.  The JNA version, 3.2.7, is the
same version used by Cassandra 1.2.2.

Added Guava 13.0.1 as a dependency.  Cassandra 1.2.2 needs 13.0.1 but
erroneously depends on 12.0:
https://issues.apache.org/jira/browse/CASSANDRA-5364

Added jsr305 as a dependency.  A few classes in titan-cassandra import
javax.annotation.Nullable and use the @Nullable annotation.  Guava
12.0 provided jsr305 to make this work, but 13.0.1 no longer provides
it.