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
