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/script/dom/create.rs

@@ -86,7 +86,7 @@ pub fn create_element(name: QualName,
 
     let prefix = prefix.map(|p| DOMString::from(&*p));
 
-    if name.ns != ns!(HTML) {
+    if name.ns != ns!(html) {
         return Element::new(DOMString::from(&*name.local), name.ns, prefix, document);
     }