Add option to set fuses without burning a bootloader

When using ICSP programmers e.g. usbasp, you can burn sketches directly
to the chip without having to burn a bootloader, however you do need to
set fuses e.g. if you're changing speed/BOD.

Updated help text to include "make set_fuses" and also changed
"make burn_bootloader" help text which wasn't entirely accurate.

Fix #141