Add Accessibility statement to pages/repository

Accessibility statement text from https://docs.google.com/document/d/1rlolXoqUv840NbuIZ0FiqRUR3NBTqiZtDMDlcxFQYEc/edit
VPAT hyperlink is currently a dead-end.
Change 'Accessibility Statement' to 'Accessibility'
Merge branch 'dryad-master' into fb6686
Fix empty label around field_filter in 'Advanced Search'

Adds an accessibly-hidden span in the label to an otherwise empty label.
Added VPAT (PDF) and other accessibility fixes.

The link to the VPAT is itself accessible. All pages with PDF files now have an (accessibly hidden) paragraph linking to the Adobe Reader app.  The VPAT notes that some archival media from the 2013 Membership Meeting has not been transcribed, nor do we have links to all required viewers.
Unrelated typo in title of Membership Meeting 2013 page.
Replacing &reg; entity with numeric equivalent.
Fix xhtml syntax in repository.xhtml
Update Last Revised date in repository.xhtml
Merge pull request #410 from datadryad/fb6686

Accessibility fixes