Add _single_path method for single resources path

It makes sure that resource id is not None, so that e.g. DELETE request
is not accidentally called on a collection URL.

Also _path method stub is now in base.Manager, to be overriden by child
classes.
Rack delete
Use _single_path in managers' get methods
Merge pull request #8 from jistr/ftr/rack_delete

Rack delete and safe _single_path helper