Add oracle registration to simple message bus controct

[brian-lc]

Overview

As a contract so that I can receive messages from the message bus, create a way for me to register myself such that I get callbacks when certain messages/message types/topics arrive.

Reference

• Mural
• Previous Issue

Questions

• How many per bus are supported?
• How are callback contracts stored?

Assumptions

• There is a way to register and de-register oracle contracts
• Events that come in on the message bus are also sent to these contracts

Acceptance

• it can register a contract to receive messages on a topic for a certain length of time
• it de-registers contracts that are not called/or are expired
• it allows for manual de-registering for certain contracts