Implement limited CSS property background, can only do colors (though this is desired behavior).

git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@332 48356398-32a2-884e-a903-53898d9a118a

TODO

@@ -6,12 +6,10 @@ Ongoing
  - Plugins for major CMSes (very tricky issue)
 
 1.0 release
- - Limited shorthand CSS attributes
-    - background
-    - list-style
- - Revise (HTML|CSS)Definition and Config relationship (groundwork for 2.0)
+ - Limited list-style shorthand CSS support
  - Lossy alternate character encoding support (characters not in the encoding
    will get silently dropped).
+ - Revise (HTML|CSS)Definition and Config relationship (groundwork for 2.0)
 
 1.1 release
  - Directive documentation generation
@@ -25,6 +23,8 @@ Ongoing
 
 1.3 release
  - Make URI validation routines tighter (especially mailto)
+ - More extensive URI filtering schemes
+ - Allow for background-image and list-style-image (see above)
  - Distinguish between different types of URIs, for instance, a mailto URI
    in IMG SRC is nonsensical