split apart dependencies of vaccine peptide selection

* added unit tests for individual ranking components
* vaccine peptide sorting by four criteria (mutant epitope score, negative wildtype epitope score, number mutated residues, distance of mutation start the edge of vaccine peptide)
* print and write FASTA of vaccine peptides with a little mutation metadata
* print epitopes associated with each mutation
Ha, I've been trying to go mostly with double quotes. The community seems to be OK with vague distinctions (e.g. symbols versus data), maybe we could enshrine something like that?

(e.g. http://stackoverflow.com/questions/56011/single-quotes-vs-double-quotes-in-python)