websearch/bibformat: metadata in header of hd records

• Metadata suitable for google scholar harvest exposed in headers of detailed record pages. see:

http://scholar.google.com/intl/en/scholar/inclusion.html

• websearch medified to emit meta tags in headers, and regression test for metadata header
• Relies on configuration within tags table in DB, but uses a minimal set of defaults that are generally acceptable. HDM format can be modified, or tag names changed.
• invenio.conf variable introduces to control which format is used (default HDM)