Allow users to specify owner names (not just ids) in excel case importer

Specifying owner names works just like specifying owner ids. A user can assign ownership to users or groups by assigning one of his or her excel columns to the special "owner_name" case property. If an owner_name and owner_id are present, the owner_name will take precedence.

Adresses this ticket: http://manage.dimagi.com/default.asp?115347
`id` will not be defined and this line will raise `NameError` if `CouchUser.get_by_username(uploaded_name)` raises `NoResultFound`.