RxScala: Change "multicast" to be consistent with RxJava

Since RxJava adds `ConnectableObservable<R> multicast(Func0<? extends Subject<? super T, ? extends R>> subjectFactory)`,  we can change `def multicast[R >: T](subject: rx.lang.scala.Subject[R])` to `def multicast[R >: T](subject: => rx.lang.scala.Subject[R])` in RxScala.

/cc @headinthebox, @samuelgruetter
/cc @benjchristensen considering this is a breaking change, please merge it before releasing 0.20 if possible.