Verify DOIs before lookup; check crossref API

@eseiver: two quick extensions that could be made to the work we just did at the hackathon:

- could clean and verify DOI numbers passed in. Eg, I think DOIs should always be all lowercase (`the_doi.lower()` function) and start with `10.` (`the_doi.startswith()` function). Or you could use the `re` regex library with a regex from stackoverflow or elsewhere to verify
- could call the Crossref API to get additional DOI metadata and inject that into the JSON object returned. I think you already have code to do this? Would probably add dependencies (`requests`, etc), which if few are ok, but if more might require a virtualenv or conda environment setup

Locate the existing DOI lookup and JSON-returning code referenced in the issue, then inspect how DOI input is currently handled and whether Crossref access already exists. Clarify the intended validation behavior, metadata fields, and dependency setup before changing anything. Done means both requested extensions are implemented and the existing DOI lookup still returns valid JSON.

Written by the indexing model from the issue text.