Operators: Observables from functionals (Action0, Func0, Runnable, Calla...

...ble)

I've created 4 operators that turn ordinary action/function calls into Observables, but unlike start and Async, the functions are called on each subscription instead of only once. They can be thought of a more generalized from(T value). Each method is named according to the accepted type to avoid overload issues with Java 8 & various dynamic languages.

The 4 additional overloads lets the user specify the scheduler where the function is called. By default, the functions are called on the threadPoolForComputation.

I don't know if there are Rx.NET equivalents of these (or they ever existed).