Added AWS SNS Notification Gateway

AWS SNS allows users to send 100 free SMS messages per month, with
pricing being competitive after that.  This gateway allows a user to
define a 'sns' media type, containing a AWS Topic ARN.  Notifications
passed by the notifier with will then get delivered to the specified SNS
topic.
Updated contact spec to allow for SNS notifications
Reverted changes to env.rb

I'm still unable to get _any_ features to run locally (Debian Wheezy,
ruby 1.9.3, redis 2.8 from source) with this line left in. Not sure why,
but evidently it makes the travis builds fail.
Merge pull request #602 from clarkf/master

Added AWS SNS Notification Gateway