Pull call number from XML holdings, not MARC record

Pull call number from XML holdings, not MARC record

KCLS uses call numbers in the MARC record, but most other sites
use the call numbers from asset.call_number. For now, just display
the first returned call number; eventually the display will probably
need to be adjusted to handle call number / shelving location / library
/ status like the current AjaxPAC does.

Also note that ##URI## call numbers should be filtered out but are
not yet.

Signed-off-by: Dan Scott <dan@coffeecode.net>