APNs feedback feature.

Wrapped the APNs socket calls to connect to either the push service or
the feedback service. Wrapped the socket call with a function to parse
the output.

Added a prune_devices management command and admin API. The management
command deactivates any device that's no longer receiving push
messages.  The admin API only deactivates the selected non-responding
devices.  These both call the get_expired_tokens free function models.py.

Added an Administration section in the readme for the feedback service API.