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
Lots of inconsistency between ' and ".

"Be consistent with your choice of string quote character within a file. Pick ' or " and stick with it. It is okay to use the other quote character on a string to avoid the need to \ escape within the string. GPyLint enforces this." https://google-styleguide.googlecode.com/svn/trunk/pyguide.html (don't see stuff about this in PEP?)

I've been going with all single quotes except for docstrings.