This branch uses EventKit, an ultra-simple library for implementing
events. The object implementing the methods maintains its own emitter
object rather than doing a mixin like Emissary encourages. This will
make it easier for us to deprecate ::on on the object itself. Unlike
emissary, the EventKit Emitter implements a super minimalistic API that
only allows one value to be emitted and always returns a Disposable
from subscriptions.