bug 2001, 2432: improve unAPI implementation

bug 2001, 2432: improve unAPI implementation

Improves Koha's unAPI support as follows:

* The implementation no longer depends on there being
  an accessible Zebra or SRW server.  Instead, responses
  are created by running the bib MARCXML through local
  stylesheets.
* unAPI now works for all bibs, not just ones that have
  an ISBN.  The ID format is changed from koha:isbn:ISBN
  to koha:biblionumber:BIBNUM.
* unAPI now correctly advertises the formats it supports.
* This implementation now passes validation testing at
  http://validator.unapi.info/ .
* MODS3 support now works correctly.  As a consequence, this
  patch fixes bug 2432 (Zotero support when using the XSLT
  OPAC stylesheets).

TODO: as additonal XSLT stylesheets are created to convert
UNIMARC bibs to additional formats, the stylesheet map
in opac/unapi should be updated.

Signed-off-by: Galen Charlton <galen.charlton@liblime.com>