Update string_cache to 0.2.

Updated string_cache, html5ever, xml5ever and selectors in Cargo.toml files and Cargo.lock.
Removed references to string_cache_plugin.
Import atom! and ns! from string_cache.
Replaced ns!("") by ns!().
Replaced ns!(XML) and co by ns!(xml) and co.
Replaced atom!(foo) by atom!("foo").
Replaced Atom::from_slice by Atom::from.
Replaced atom.as_slice() by &*atom.

components/layout/query.rs

@@ -555,7 +555,7 @@ pub fn process_resolved_style_request(requested_node: ServoLayoutNode,
         }
         // FIXME: implement used value computation for line-height
         ref property => {
-            style.computed_value_to_string(property.as_slice()).ok()
+            style.computed_value_to_string(&*property).ok()
         }
     }
 }