Add requestFromPubMedID, requestJSON

requestFromPubMedID can take a PMID, PMCID, mauscript ID or a versioned identifier. It
finds the underlying DOI with PMC's API[1] and requests citation metadata by passing
that to requestFromDOI.

requestJSON is a helper function that fetches and parses a remote JSON endpoint.

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

Bug: T1197
Change-Id: I1852f9820d9a0e1af0f30003d84846b435095e16