Implement and test clock commands

[arnsholt]

Commands from code to the clock are the last missing major piece (along with EEMoves, #2 ) of functionality before the code should be more or less ready for a 1.0 release.

First, I need to implement the stuff, including ACKs from the clock, which are honestly a bit weird, and then test it with an actual board and clock. For that last I should probably write a small program that exercises all the relevant functionality.