A minimal Universal Spectrum Identifier parser and PROXI client

[mobiusklein]

This PR implements a minimal Universal Spectrum Identifier parser and PROXI client to consume USIs and return spectra from Peptide Atlas, MassIVE, PRIDe, and jPOST.

[levitsky]

Thank you! Looks great.

Looking at the test, though, I wonder if there's an easy way to also test the functions without numpy.
Also, not sure how much we should care about Python 2 compatibility at this point, but the <= comparison on dictionary keys seems to have a different meaning there. The tests pass as long as the dicts are equal, of course.

[mobiusklein]

You're right, on Py2 dict.keys isn't Set-like. I've fixed that, and removed the dependency on numpy from the test too.

[levitsky]

Ah yes, this is surely the easiest way. I was probably trying to think of something too complex. Thanks again!