revamped spawn to use event-based impl by default

this patch turns libcppa's spawn function family upside down;
actors are now event-based by default, but users can opt out
by using `spawn<blocking_api>(...)`;
this patch also removes the old `scheduling_hint` interface and
replaces it with the more versatile `spawn_options`, e.g.,
`spawn_monitor<detached>(...)` becomes `spawn<monitored + detached>(...)`