Now counts all subscriptions, including internal
Now counts all subscriptions, including internal

* Almost a complete rewrite
* Subscribing to an observable creates a new observable which increments and decrements a global counter when it is subscribed to/unsubscribed from.
* Uses a pollyfill of WeakMap to cache all the new observables which are created.
* Prints the number of subscriptions using a throttle instead of on an interval so that it reponds to changes sooner.
acutmore/leaks