Added functions to calculate codon distance

Added a function that takes two amino acids and print the nucleotide distance for all possible codon combinations.
s/.keys()//

You can just take the `len` of a `dict` to see how many keys there are in it. Calling `keys()` actually goes and makes a list, which you don't need to do since a `dict` already knows how many elements it has in it.

You should re-read the Learning Python book. You'll fill in some gaps like this and it will make your Python knowledge more solid. (I've read it at least twice, and some chapters 3 or 4 times. There's still stuff I don't know.)