Make use of the simple NCBI ID converter API

[emanuil-tolev]

http://www.ncbi.nlm.nih.gov/pmc/tools/id-converter-api/

Currently we're doing PMID -> DOI lookups by looking up the whole item like

xml_url = "http://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.fcgi?db=pubmed&id=" + canonical_pmid[5:] + "&retmode=xml"