Move connection initialization to base class.

Moving common connection initialization to AbstractConnection allows other connection classes to take advantage of it, avoiding duplication.
Add Apple Push Notification Feedback Service.

* Added feedback service URLs to ConnectionInterface.
* Connection\Feedback handles creation of socket connection.
* Basic FeedbackException
* Feedback\Service reads invalidated device tokens from the stream and returns an array containing instances of Feedback\Device.
* Feedback\Device is a simple class to hold invalidated device data.
Unit tests for Feedback connection, Device, and Service.