Fixes and new Features

• Breaking Change: BehaviorSubject now has a separate factory constructor seeded()
  This allows you to seed this Subject with a null value.
• Breaking Change: BehaviorSubject will now emit an Error, if the last event was also an Error.
  Before, when an Error occurred before a listen, the subscriber would not be notified of that Error.
  To refactor, simply change all occurences of BehaviorSubject(seedValue: value) to BehaviorSubject.seeded(value)
• Added the groupBy operator
• Bugix: doOnCancel: will now await the cancel result, if it is a Future.
• Removed: bufferWithCount, windowWithCount, tween
  Please use bufferCount and windowCount, tween is removed, because it never was an official Rx spec.
• Updated Flutter example to work with the latest Flutter stable.