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
Instead of adding the local_name of an attribute, if a prefix exists,
add prefix:local_name to allow users to retrieve it via the namespaced
key name.
For example, with this XML:
```
<?xml version="1.0" encoding="utf-8"?>
<root xml:lang="en" >
</root>
```
`root.attr("xml:lang").unwrap()` will now correctly return "en".
`root.attr("lang")` will not retrieve "xml:lang" value anymore.
This is a breaking change.
Fixes#2
