Fix code style minorly

Found some trailing spaces and indentation issues.
Simplify handler API to remove handle method

Previously, handlers (including senders) were objects with a
".handle()" method which took a request and return a response. Now,
handlers (again including senders) are callables which themselves take
a request and return a response.

This will simplify the process of writing very simple handlers. Some
users may well be best off writing handlers actually in their
settings.py as a simple function rather than a callable class instance
or some such. This new API should make such usage simpler.