Added a function to generate nonce via /dev/urandom on Unix. This is a fix for the problem
that (random most-positive-fixnum) with SBCL 1.1.6 on OS X does not create enough randomness.
The nonces start repeating after less than a dozen attempts.
Merge pull request #16 from MadEarl/master

nonce generator for Unix based on /dev/urandom