Support raw Notifications

[gaffneyc]

SNS will send the full message as the body of the notification and include some information as header fields. Will need to look for the x-amz-sns-rawdelivery header being set to true.

Main question is: should raw messages be treated as binary? Or are they still strings? If they're strings we can just the Message field otherwise I'm tempted to add a Raw() ([]byte, bool) method that returns the raw data and true for raw messages.