0.25.0-beta

• stream transformations on a ValueStream will also return a ValueStream, instead of
  a standard broadcast Stream
• throttle can now be both leading and trailing
• better handling of empty Lists when using operators that accept a List as input
• error & hasError added to BehaviorSubject
• various docs updates
• note that this is a beta release, mainly because the behavior of transform has been adjusted (see first bullet)
  if all goes well, we'll release a proper 0.25.0 release soon