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/gfx/platform/macos/font_template.rs

@@ -57,7 +57,7 @@ impl FontTemplateData {
                         Err(_) => None
                     }
                 }
-                None => core_text::font::new_from_name(self.identifier.as_slice(), 0.0).ok(),
+                None => core_text::font::new_from_name(&*self.identifier, 0.0).ok(),
             }
         }
         ctfont.as_ref().map(|ctfont| (*ctfont).clone())