Optionally log and replay serial commands sent to EBB

[RaphaelWimmer]

These commits enable ebb_serial.py to optionally log all commands sent to the EBB into a file. A script ebb_replay_log.py allows for replaying the list of commands to a serial port.

Together with changes to the EggBot Inkscape extension, this makes it possible to create a design in Inkscape, save the plotting commands into a command log, and then plot the design again without requiring Inkscape. This is useful for automated / repeated plotting of the same design.