replaced notifier.js with fxa-notifier-aws

This removes the notifier.js pipe-chained process
and the direct aws-sdk dependency and replaces them
with the fxa-notifier-aws module. When an snsTopicArn
is set in the config the process will directly send
events to SNS.
Please replicate this comment into the fxa-notifier-aws source, I had forgotten what the setTimeout thing was all about when I saw it over there.