Prevent closing and re-opening the serial port several times, fixes issue 16 (and probably 18)

[rreilink]

Instead of opening and closing the serial port for every step, do it only once. This solves the issues related to the arduino resetting in between, and speeds up the programming since each resets costs a few seconds (may be depending on the Arduino type)

With this PR, the changes related to the 'noinit' of the Arduino gal variable could be reverted; I can make a commit for this if desired.