optimizing port fetching with _difference
AVR109 bugfix for repeated re-flash
AVR109 execute callback only once

if any new bytes arrive afterwards (and next callback wasn't set), those
bytes get ignored/dropped on next .send command
AVR109 re-initializing instead of resetting vars
STK500/AVR109 verification bugfix
AVR109 now uses "on the fly" verification
removing non-needed permissions
enabling text-select in log (Christopher Cope)
adding proper documentation for 250 B delay
bumping to v 0.6