Detects and treat links to ebooks as enclosures

Request here : http://forums.cocoaforge.com/viewtopic.php?t=26520&p=138182
OPDS uses a variation of Atom feeds, with acquisition links : http://opds-spec.org/specs/opds-catalog-1-1-20110627/
Treat relative links for enclosures.

Related to http://forums.cocoaforge.com/viewtopic.php?t=26520&p=138182
Merge pull request #275 from barijaona/OPDS

OPDS support