Build in support for going from a DOI to a PMID

[JimHokanson]

Eventually we should be able to go from a DOI to a PMID. Unfortunately this functionality is primarily provided by Pubmed, not Crossref.

A Pubmed API exists in BioPython, but BioPython itself is really heavy. For now it might be best to just place a few lines in here that implements this functionality.

This is an example call:
http://www.ncbi.nlm.nih.gov/pmc/utils/idconv/v1.0/?tool=my_tool&email=my_email@example.com&ids=10.1093/nar/gks1195

Notice you need to provide the name of your tool, your email, and then you provide the DOIs to resolve.

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

Even better:

http://www.ncbi.nlm.nih.gov/pmc/utils/idconv/v1.0/?tool=my_tool&email=my_email@example.com&ids=10.1093/nar/gks1195&format=json

Multiple ids should be comma separated:
service-root?ids=PMC3531190,PMC3245039