8263 Consistent stylesheet prefs

8263 Consistent stylesheet prefs

This patch makes the use of opaccolorstylesheet and opaclayoutstylesheet more consistent. They may be: 1) just a file name, 2) a complete local path or 3) a full URL starting with http: for a remote css file.
This makes the syspref opacstylesheet that was only used for a remote css file obsolete.

June 20, 2012 Rebased.
July 18, 2012: Regex allows https too (thanks to Owen Leonard).

Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>