Added v2/event_subscriptions endpoints.
Supported methods:
  POST with ?callback_url
  DELETE with ?callback_url

Now, we cannot return any value on subscription endpoints  Because Guava's event bus is totally one-way, which means Guava's event bus  can't return values.  To provide event_subscriptions endpoint, we probably would have to re-think architecture about this so that it doesn't use Guava's event bus.
cleaned up internal codes in HttpEventActor.
/v2/event_subscriptions now has returns result to client. and support GET method on this path to get the list of subscribers.

eliminated a dependency between EventSubscriptionsResource and event bus and Introduced HttpCallbackSubscriptionService instead of it.