Amazon device messaging

Hi, I added support for Amazon Device Messaging to work with Kindle devices.  Currently, ADM requires a POST request for each notification to be sent to a device.  They are working on functionality to support bulk notifications that can made with one request.  Once they roll that out, I will update accordingly.

I also added rapns:notifications:clean to allow completed rapns_notifications to be cleaned up.

Specs can be found in their respective adm directories. Please let me know if there is anything else I may have missed.  Thanks!