ConnectableObservable autoConnect operator

[akarnokd]

There is a need for auto-connection capability for an arbitrary ConnectableObservable similar to how it is possible now to use cache() as an auto-connected unbounded replay.

The new autoConnect() method let's specify the number of Subscribers required for the real connection to happen and also offers the ability to receive the connection Subscription.

I've also updated the parameter name of cache(int) to reflect it is a capacity hint and not a bound.

[benjchristensen]

autoConnect is a nice additional feature. Like refCount except it never disconnects. I particularly like the overload where it auto connects when the specific number of subscribers arrive.