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.
add category to aps_data dict
Support Django 1.7 migrations.
Improve the travis matrix
Fix tests on 1.7+
Fix broken tests on 3.x
Get username field dynamically in the admin

Fixes #70
Fix APNS_FEEDBACK_HOST url

Fixes #85
Fixed the issue with production and sandbox addresses for APNS Feedback Service beind mixed up
apns: Make APNS_MAX_NOTIFICATION_SIZE a setting

Also bump the limit from 256 to 2048 due to the increase at iOS8's release.

Fixes #79
fix `HexIntegerField.get_prep_value` when value is already a string

value may already be an integer so trying to create a base 16 int from
it fails
Change SSLv3 to TLSv1
apns: Fix bad reference to max notification size

Fixes #93
Thanks, Jay Camp
README: Fix an unexpected indentation error
README: Remove some badly-parsed indents
fix APNS test after introduction of APNS_MAX_NOTIFICATION_SIZE
added support for GCM time_to_live parameter
Release v1.2.0
cleanup unnecessary return and redundant error checking
handle timeout in apns feedback socket with python 3