Minor cleanup and Question 67.

Answer to 67 seems more efficient compared to most
solutions that people pick, still not fast enough IMHO.

Correction of 67 after my inefficient implementation of
http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes.

My previous impl required a max number value for the list of
primes, I could not make it both incremental and fast enough.