Added function to return all translations of a DNA sequence

I've added 6-frame translation to AA, along with tests.  For now I'm not sure what the best place (in the code) to extract ORFs is, so I've left it. There's a tiny throwaway function we can use to get (a lower bound on) the length of the longest ORF in an AA sequence.

Fixes #219 

Yes, maybe you're right (you probably are).  I wanted some dumb function so I could get a number out in order to write/run the dna-to-aa.py script.  I'll think more...