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.

More cleanup and documentation for APNS feedback

 - Added an “Administration” section in the readme for the feedback
service API
 - Moved the “get_expired_tokens” function from APNSDevice to a free
function (still in models.py)
 - PEP8 formatting.
Merge branch 'master' of https://github.com/shigmas/django-push-notifications