Allow Supervisors to be stopped

[hoodie]

Hi there,
as described in #27 it would be really nice if Supervisors could be stopped.
I would like to propose to add the .stop_supervisor() method to Addr and Context to achieve this.
In this PR I did the following.

1. add another ActorEvent called ActorEvent::StopSupervisor which is ignored by normal Actors but obeyed by Supervisors.
   You can now call .stop() to stop the Actor (which will lead to a restart by the supervisor) and .stop_supervisor() which will also stop the Actor but inform the Supervisor to not restart.
2. Modify the existing example/supervisor_clear_interval.rs
3. also: I noticed that the Supervisor::start method looked very similar to the Actor::start implementation and since there was already the ActorManager I opted to move them both into the same thing. I took the liberty of renaming it to lifecycle, as the name maybe a bit more familiar.

this fixes #27

Thank you

[hoodie]

@sunli829 what do you think?

[hoodie]

@sunli829 friendly ping

[hoodie]

@sunli829 another friendly ping, I know these pass unnoticed sometimes on github.
If you're not interested in this crate anymore, I'd be willing to take over maintainership btw.