Change override mechanism 

Perhaps we should change the way `override` works. Right now it will override a rule if the regexp or the name of the field are the same, but probably it is better to be a bit more explicit, i.e.:

``` python
@marc21.over('image_url', '^8564_', override='electronic_location_and_access')
@utils.for_each_value
@utils.filter_values
def image_url(self, key, value):
    ....
```
