Use fast sieve of Eratosthenes to generate primes

This results in a 30x speedup of the prime generation for
every time crunch is loaded. (Previously ~10ms, now ~0.3ms).

See http://jsperf.com/prime-generation/2 for a speed comparison
of different ways to compute primes.
Merge pull request #7 from swenson/fast-primes

Use fast sieve of Eratosthenes to generate primes