Adds some tests for the "unavailable" state
Refactors ServiceActor to inject TokenService ref

This commit refactors the ServiceActor such that it takes an ActorRef to
the TokenServiceActor. Adds tests to verify that a token is requested.
Refactors out the TwitterService actor ref
Splits out separate spec for the ServiceActor

Didn't seem to make sense mixing the Actor testing with the RouteSpec.
Still not 100% the Actor tests are right...