Citoid: Add requestFromPMID() method to lib/request.js
completed by: ☃ unicodesnowman
See https://phabricator.wikimedia.org/T1197 for the corresponding bug report. In the file lib/request.js, currently there are only methods to get metadata from a journal paper from its URL or DOI. However, you can also find information using another identifier, PMID. You can use the PubMed API to find a paper's DOI given its PMID and therefore its url: http://www.ncbi.nlm.nih.gov/pmc/tools/id-converter-api/
Students are required to read Wikimedia's general instructions at https://www.mediawiki.org/wiki/Google_Code-in_2014#Instructions_for_GCI_students first.