Monome-style I2C implementation

[infovore]

A number of tasks need solving here:

• follower mode (how do we do this?)
• leader mode (how do we do this?)

Is there anything to be gained from the txhelper libraries, etc?

And: how do we test this?

[infovore]

• [x] add callbacks for follower mode
• [x] investigate TxHelper for follower mode
• [ ] add master mode functionality

[infovore]

i2C Master is it's own thing; for now, we have I2C Leader working nice and simply.