Small change to allow predicates to be Python Dotted Names

This allows a {view,route,subscriber} predicate factory to be named as a
python dotted name that is then automatically resolved.

Includes tests.
Update documentation to reflect the dotted python name
Deprecate custom_predicates for add_view/add_route
Remove useless spaces.
Add feature update to CHANGES.txt
Merge branch 'bertjwregeer-feature/allow_dotted_predicates'