layout: Implement basic lists and the CSS1 list properties.

The exact rendering is ill-spec'd. Some things are ugly (especially the
width and height of list style images) but they are infrequently used
and I believe this implementation matches the spec. Numeric lists are
not supported yet, since they will require a separate layout pass.

The implementation is a subclass of `BlockFlow`, on advice from Robert
O'Callahan.

components/layout/table_cell.rs

@@ -23,7 +23,7 @@ use sync::Arc;
 /// A table formatting context.
 #[deriving(Encodable)]
 pub struct TableCellFlow {
-    /// Data common to all flows.
+    /// Data common to all block flows.
     pub block_flow: BlockFlow,
 }