Fix the PartialEq implementation for Element

The order of attributes in an `Element` doesn't matter anymore.
`<elem a="b" c="d" />` and `<elem c="d" a="b" />` are now correctly
considered equal.

For that I had to derive `PartialOrd` and `Ord` for `Attribute`.
This allows us to sort cloned vectors of `Attribute` in the `PartialEq`
implementation and compare them instead of the struct `attributes`.

Fixes #3

src/attribute.rs

@@ -9,7 +9,7 @@ use std::fmt;
 /// This is of the form: `name`="`value`"
 ///
 /// This does not support prefixed/namespaced attributes yet.
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq, Ord, PartialOrd)]
 pub struct Attribute {
     /// The name of the attribute.
     pub name: String,