Add parsing for mutant alleles

[iskandr]

IEDB is full of experiments with alleles such as HLA-DRA*01:01/DRB1*01:01 C30S mutant, one way to support this is to add a mutations field to AlleleName which is a list like ["C30S"], sorted in order of the protein positions. The mutations can also be represented with more structure via a class like:

class Mutation(object):
    def __init__(self, aa_ref, aa_mut, aa_pos):
        self.aa_ref = aa_ref
        self.aa_pos = aa_pos
        self.aa_mut = aa_mut