context can now be given as argument in StyleMap.addUniqueValueRules()

this, for example, allows user to access to properties that are not part of the feature attributes.
r=elemoine (closes #1548)

git-svn-id: http://svn.openlayers.org/trunk/openlayers@7216 dc9f47b5-9b13-0410-9fdd-eb0c1a62fdaf

lib/OpenLayers/Rule.js

@@ -173,6 +173,9 @@ OpenLayers.Rule = OpenLayers.Class({
         if (!context) {
             context = feature.attributes || feature.data;
         }
+        if (typeof this.context == "function") {
+            context = this.context(feature);
+        }
         return context;
     },