Add RefCount overloads with minimum observer count option

[akarnokd]

This PR adds overloads to the RefCount operator that allows specifying the minimum number of observers to subscribe to it before the connection is established.

The change is trivial in the operator replacing the original constant representing one observer with an operator parameter.

var source = Observable.Range(1, 5).Publish().RefCount(2);

source.Subscribe(Console.WriteLine);
Console.WriteLine("No values should be printed yet");
source.Subscribe(Console.WriteLine);

prints

No values should be printed yet
1
1
2
2
3
3
4
4
5
5

Resolves #406