Implement Diceware Passphrase Generator

Note: This is a first swing. I do not have the hardware necessary to test it properly at the moment.

Fixes #163 
I didn't notice this on the first read-through, but this line does rejection sampling to deal with the bias issue I mentioned previously. It needs to be adjusted for the new bigger range, though. I'd suggest generating the value first (i.e., byteArray[0] * 256 + byteArray[1]), then checking to see if it's above the highest round-number multiple of range that fits in max_range. Once that's done you can remove the TODO.