kohabug 2078 - fix feed validation errors

kohabug 2078 - fix feed validation errors

Fix several validation errors in RSS and Atom feeds
generated from the OPAC:

- add missing guid to RSS elements
- add missing feed ID and element ID to Atom elements
- reflect OPACBaseURL changes
- fix atom:link self links
- add HTML escaping to fields comming from bib record
- set default timestamp for Atom updated elements

Issues identified but not solved in this patch:

- setting Atom updated element for each bib (presumably
  from biblio.timestamp)
- possible problems performing paging of Atom feed

Based on successful validation of feeds by feedvalidatory.org,
it is expected that feeds should now work in IE7 and IE6.

Signed-off-by: Joshua Ferraro <jmf@liblime.com>