Fixed titan-cassandra pom.xml typo

<scope>optional</scope> is not valid.  I intended
<optional>true</optional>.  This changed accordingly.
Update guava dep and add jsr305 dep

Increased Guava version from 12.0 to 13.0.1 to harmonize it with
Cassandra 1.2.2's Guava 13.0.1 dependency.  Also declared the jsr305
dependency (provided implicitly by Guava 12.0 but not Guava 13.0.1) to
support @Nullable use in the source tree.