Improve information retrieved about DOIs

[Mihai Glonț]

Use http://search.crossref.org/help/api and http://search.datacite.org/ui to retrieve metadata about a publication based on its DOI.

[Camille Laibe]

EuropePMC also allows query based on DOIs (for publications which also have a PMID or PMCID). For example, if you use their JSON, you can use URLs of the form: http://www.ebi.ac.uk/europepmc/webservices/rest/search/query=DOI:10.1093/nar/gkr1097&resulttype=core&format=json