Don't require setters for events in AnyObservable
Make parameter names in EventSubscription.init required
EventSubscription: free handler on invalidation
EventSubscription: make sure owned subscriptions are invalidated
Merge branch 'master' into chaining
ChainingProxy: allow chaining with non-optional path
Add tests for ChainingProxy: structs, optionals, owner, cleanup
ChainingProxy: make forward references weak, add strong back references

TODO: cleanup