Function to summarize bases at a given position in a fasta file

Added a function to take a fasta file with sequences and a position and return information about which bases / amino acids are found at that position.
Sorry to do it on my master, I got over exited...
See `from collections import Counter`  https://docs.python.org/2/library/collections.html#counter-objects
(I've only just started to use them myself).