API Improvements

• 2016.03.02 - add implicit execution context parameter to: all Repo
  methods; RepoPool.createMany; and LongevityContext.repoCrudSpec
  and inMemTestRepoCrudSpec. users now need to provide execution
  contexts to use all these methods. the easiest way to do this is to
  include import scala.concurrent.ExecutionContext.Implicits.global
  at the top of the file.
• 2016.03.08 - update to latest version of library dependencies
  casbah (3.1.1) and cassandra (3.0.0).
• 2016.03.08 - add sub-projects longevity-cassandra-deps and
  longevity-mongo-deps.
• 2016.03.10 - replace Root with Persistent. give Persistent
  three child traits: Root, ViewItem, and Event. these changes
  should not affect existing code that uses Root.
• 2016.03.10 - replace RootType with PType. give PType three
  child traits: RootType, View, and EventType. these changes
  should not affect existing code that uses RootType.
• 2016.03.25 - rework PType API for keySet and indexSet. please
  see the latest documentation for a review of the new API.