refactor DOI finding.
refactor DOI finding.

This adds support for finding DOIs from CrossMark tags, and fixes some
bugs in how we were pulling them from Altmetric widgets (fixes #6). It also
refactors the whole thing to make it easier to grok the process and easier
to add new ways of finding DOIs.

Impactstory/unpaywall,Impactstory/unpaywall,Impactstory/unpaywall