Explanation of 'wheel' data on primes.clj, and version which generates wheel from initial primes.

The 'wheel,' which is the centrepiece of primes.clj, is unfathomable (to me and others) without exposure to O'Neill's paper "The Genuine Sieve of Eratosthenes."

I have added a reference to the paper, and an explanation of the rationale of the wheel, to the primes example.  I have also included another version which generates the wheel from the initial vector of primes, which allows for experimenting with length of the initial vector of primes.