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 

This returns the length from stop codon to stop codon. Shouldn't it be start to stop codon? I realise this may be a can of worms.