TWI: Improve asynchronous mode

[mikaelpatel]

Current implementation will synchronize with driver on twi.end(). An Event handler is possible but could be a callback to the driver instead which would be responsible for events in asynchronous mode.

The TWI::Driver class would be extended with a new virtual member function on_completion() which is called by the TWI state machine. The default implementation would do an event push if a handler is defined.

[mikaelpatel]

Initial refactoring; see commit 173c3eb.