Added cassandra store

Cassandra support cannot be implemented as a new store adapter, as
cassandra is not a key-value store.  We now have a Cassandra-specific
store.
Added cassandra cluster runner

Only runs on the default port - so integration tests
cannot be parallelized any more... changing this will involve
diving into Cassandra's config.  Which is not fun.
removed travis.yml -- tests are all run on hm-workspace