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/htmlfieldsetelement.rs

@@ -87,7 +87,7 @@ impl VirtualMethods for HTMLFieldSetElement {
     fn attribute_mutated(&self, attr: &Attr, mutation: AttributeMutation) {
         self.super_type().unwrap().attribute_mutated(attr, mutation);
         match attr.local_name() {
-            &atom!(disabled) => {
+            &atom!("disabled") => {
                 let disabled_state = match mutation {
                     AttributeMutation::Set(None) => true,
                     AttributeMutation::Set(Some(_)) => {