Exclude HBase from TitanGremlinPlugin

The plugin only includes titan-core and titan-cassandra and their
dependencies in this commit.  Excluding HBase for the experimental
zipfile to avoid potential complications, e.g. HBase dependency
versions conflicting with those used in
gremlin-console/giraph-gremlin/titan-cassandra/titan-core.
Merge branch 'titan09' of github.com:thinkaurelius/titan into titan09
Refactoring Cassandra input formats

This commit also adds some comments to ScanJob with information about
method contracts inferred from the current executor implementation