Phrase generator now creates alphanumeric codes rather than naughty phrases.

e.g., 'nc8K-ppBP-eB9z-r9t9' instead of 'creepily-erect-finger'.

The alphabet used in the new "words" excludes characters that look the
same in many fonts: l, I, 1, 0, and O.

This is an alternate solution to pull request #156.