Read incoming DOI and find matching study metadata

This does a server-side query of CrossRef.org, using their API to look
for studies that match the given DOI. If it finds any matches, we take
the first one and inject its metadata (reference string, publication
URL/DOI, year) into the study.
Merge pull request #66 from OpenTreeOfLife/populate-new-nexson

Read incoming DOI and find matching study metadata