cas:serviceResponse multiple xml nodes with the same tag_name

[skudi]

Fixes #14
If cas:attributes under cas:serviceResponse contains multiple nodes with the same tag_name, their values are accessible as python List.

<cas:attributes>
     <cas:firstName>John</cas:firstName>
     <cas:role>user</cas:role>
     <cas:role>manager</cas:role>
[...]

response.attributes['role'] == [ 'user', 'manager' ]