Modify APNsConnection __del__ method

1. close the stream when it exist in APNsConnection destruction
2. add __del__ method to GatewayConnection and FeedbackConnection

	modified:   apns.py
Modify _feedback_callback method's return

Modify _feedback_callback method in FeedbackConnection, changing the return of
time from datetime to unix timestamp

	modified:   apns.py
Modify unittest

	modified:   tests.py