Add explicit alternative methods to File::on
Use explicit event subscription methods in specs
Show ::on deprecation message for unrecognized event names
Track disposal of subscriptions made through deprecated ::on as well
Replace Directory::on with ::onDidChange

- Also, deprecate the old method
Use emissary meta events to track subscription count

Trying to shim this via the ::on override is too unreliable because
there’s multiple ways to unsubscribe.
:pencil: Document event subscription methods
:pencil: Remove string-based event docs
Remove catch-all deprecation warnings

They interfere with self-subscriptions for meta events and calls with
unrecognized event names were no-ops previously.
Check subscription if count is 0 *after* decrement
Unsubscribe from native change events in afterEach

Instead of calling .off, which no longer has the desired effect. We
could also dispose every subscription, but this is easier.
Merge pull request #41 from atom/ns-event-methods

Add explicit event subscription methods