[MERGE] Command Not Found core code

[Ian Clatworthy]

Martin Albisetti wrote:

> With this code merged, I can also make a plugin available so people
> can start playing around with it easier.

This looks a step forward to me. Some minor changes needed.

bb:tweak

> +        else:
> +            raise errors.CommandAvailableInPlugin(cmd_name, plugin_metadata, provider)

This line is over 80 chars so needs to be wrapped.

> +class ProvidersRegistry(registry.Registry):
> +    '''This registry exists to allow other providers to exist'''
> +    def __iter__(self):
> +        for key, provider in self.iteritems():
> +            yield provider
> +    pass

Blank line needed between the class docstring and the first method.
The 'pass' line ought to go as well, right?

Ian C.