Allow plugins to send messages to each other

[dmccoystephenson]

It should be possible for plugins to send messages to each other. They can act as producers or consumers for a particular topic-like data structure called a Mailbox. Much like Kafka a simplified implementation will do for now, but later we may want to be able to run Kafka under the hood. This will allow for plugins to subscribe to each other's messages via mailbox and this should make integrating plugins easier.

Ideally projects would not need to add mailboxes as a dependency in order to use it. It would be better if mailbox is exposed a rest API that plugins could use to send messages.