Remove unnecessary fat arrows

Observable::flatMapError and Dispatcher::removeSub are not called
directly, so they don’t need to be bound to the instance.
Store Dispatcher unsubscribe function directly in object

Instantiating new functions in large number is costly. This drops
subscribed function memory use by 0.49KiB and makes Bacon significantly
faster on Node 0.10.
Merge pull request #457 from lautis/dispatcher-tuning

Dispatcher tuning