Add oracle registration to simple message bus controct

## 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](https://app.mural.co/t/consensys3989/m/consensys3989/1557345438344/718181f0ee878f1cfdc2756c9dc6aa363680b634)
- [Previous Issue](https://github.com/ConsenSys/web3studio-sidejam/issues/5)

### 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