(core) package shuffling.

Created dh.network package
(core) Rename type RegisterSelection -> AddressSelection
(core) rename files to match class name
(core) Rename GatewayService -> DeviceDirectoryService

Not every Device will be accessed through a Gateway
(core) prevent joda warnings
(core) Testing dependencies
(core) Start actor explicitly to ease testing.
(core) Extract testable functionality of the actor.

The only real logic of this actor is contained within how often is
attempts to poll.  So pull that logic out into a testable method that
doesn't depend upon the current time.
(core) A simple actor test
(core) Props definition for PollingActor