Message queue implementation

Put a message on the queue when content is created or updated.
@bradleywright: fair point about message queues being first class. we're queuing up content items in rabbit after_save, so i want to ensure that there's no deviation between counts of content_items saved and content_items queued. wasn't thinking about a queue backing-up another queue at all; we could flag items which couldn't get queued for some reason and chronically attempt to queue them. that's if it is content-store's concern.

if we want this to be a client concern, content item creation and queuing should be wrapped in a transaction. so that we are sure that every content item saved in the content store has also been queued for processing.