look for prism and eprints metadata ; normalized previous "scholar" key as "highwire" for Highwire Press Tags, which is what they actually are.
extract twitter card metadata, renamed "og" to "facebook", generalized _getMetaTags to work with the variations in metadata expression. fixes #214
Merge pull request #223 from edsu/212-extract-prism-and-eprints-metadata

look for prism and eprints metadata