remove RemoteNotifications module

Instead add configurations to Stethoscope and Heart and set a default
notifier name.

Increased flexibility and less coupling, plus allows for testing without
needing a remote notifier running.
terminate actors created during tests

Actors are not terminated when they go out of scope, so we have to keep
track of which ones were created during a test and terminate them.
add terminate_all(klass)

Some tests rely on there only being one of the tested actor. Though
there are before/after hooks that terminate actors created during tests,
terminate_all makes it explicit that the test relies on a singleton.
add Stethoscope specs
add Heart specs