Switched from __init__() to constructor() in Actor

to avoid a shitload of easy-to-fuckup boilerplate. Might be a bit more
counterintuitive but eh. We can always revert it.
Minor changes, nothing interesting
Added option for non-blocking main_loop in Actor
Added names to all Actors (instead of pid)
(Re)wrote the message system

([target(s)], [source(s)], payload)