Allow decider to start with just a task list

[jbbarth]

Today we force to have a "workflow" as argument to decider.start, but we should be able to start with just a task list name and find the correct workflow the same way we find activities (== resolve the workflow type as a python path to a class).

Actually this is what we do for child workflows so that should be possible and it would be easier to manage deciders fleets (less boilerplate for declaring deciders).