Added a better prime-number generator

Inspired by the Prime Factorization function I wrote, I decided to use
it as a prime-number generator.
Times for 10k Primes: 1:50s, 20k Primes: 6:50s, 30k Primes: 15:06s.
Which is better than the last function I wrote in MIT_6_00 ps1a.py which
generated 10k Primes in 4:52s.