fix dropdown mask in ie7/8. fixes #926

select2.css

@@ -120,6 +120,10 @@ Version: @@ver@@ Timestamp: @@timestamp@@
     top: 0;
     z-index: 9998;
     opacity: 0;
+    filter: "alpha(opacity=0)"; /* expected to work in IE 8 /
+    filter: alpha(opacity=0); / IE 4-7 */
+    -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=0)"; /* works in IE 8 */
+    background-color: #fff;
 }
 
 .select2-drop {