Solutions to Challenges 1, 2, and 4

Solutions to challenges 1,2 and 4.  Didn't have time to complete number 3.

Regarding #4, There are 2 weaknesses with the solution as it stands:
1. It does not safeguard against an infinite cycle, but since the Collatz conjecture has been verified up into the billions, I chose to not add handle it here for values up to a million
2. For inputs which are very far apart, this solution will fail by running out of memory.  I was hoping to handle that by calculating in blobs of 10,000 or so (parallelized maybe?!), but ran out of time.