Bounce notifications

This is a pull request for issue #44

The pull request adds a "handle_bounce" view that handles SES bounce and complaint messages via a SNS webhook.

# The View

The "handle_bounce" view accepts a SNS notification message and based on the notification type calls one two new Django signals. If the notification is a bounce message the "bounce_received" signal is called, otherwise if the notification is a complaint the "complaint_received" signal is called.

The view also accepts and  and automatically confirms subscription confirmation requests.

# Other changes

The example project hadn't been updated in a while so I updated to make it usable and so It could be used to test SNS messages.

# Signature verification

Currently the signature of the bounce/cancel requests are verified by default. This requires the M2Crypto and requests libraries to be installed but they are left out of the dependencies in setup.py as they are optional. Some kind of documentation might be needed in the future.