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.

This pull request adds ```make set_fuses``` to achieve this capability.

I've also updated the help text to include ```make set_fuses``` and also changed ```make burn_bootloader``` help text which wasn't entirely accurate (it said you can burn bootloader and/or fuses, whereas the "or" was not possible)

P.S. ```make ispload``` doesn't set the fuses anymore.