Add full Image support to channels.
 - Refactor persistence of temporal fields - move persistence methods from Channel/ChannelGroupTranslators
to the TemporalTitle/ImageTranslator
- Clean up imports on several tests
Move AstyanaxContext creation to CassandraPersistenceModule, and move cluster/keyspace properties
to environment properties instead of hardcoded constants file. N.B. the tests still have hardcoded
properties, these will need migrating to environment properties at some point.
Write new fleshed-out images to a new field, called 'new_images'. write previous data
to 'images'.