Bug 11786: Improve RDFa structured data schema.org mappings

Bug 11786: Improve RDFa structured data schema.org mappings

This patch updates both the prog and bootstrap themes to provide better
schema.org mappings for catalogue elements in the following ways:

* More granular author attributes (one "author" property per author in records
  with multiple contributors; wrap in a Person or Organization type instead of
  using a simple literal value)
* Use "byArtist" instead of "author" for music recordings to respect schema.org
  preferences
* Use alternateName instead of alternativeHeadline, as the former is a better
  parallel to the "name" property

Signed-off-by: Dan Scott <dan@coffeecode.net>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>
Only covers MARC21, but that is no reason not to move it forward

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Passes all tests. Looked at the source code of a few records in both
prog and bootstrap and took a look at the properties using
http://www.google.com/webmasters/tools/richsnippets.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
(cherry picked from commit 3bc1021deb851db8965e9ac3a8ff89aba71f6acc)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>