Add common methods and fields from Stream/Property to Watchable

[Dan Schultz]

Methods:

• any():Future
• asyncExpand():EventStream
• asyncExpandLatest():EventStream
• asyncMap():EventStream
• contains():Future
• distinct():Reactable
• delay():Reactable
• drain():Future
• elementAt():Future
• every():Future
• expand():Reactable
• firstWhere():Future
• fold():Future
• forEach():Future
• handleError():EventStream
• join():Future
• lastWhere():Future
• map():Reactable
• reduce():Future
• scan():Property
• singleWhere():Future
• skip():Reactable
• skipWhile():EventStream
• take():Reactable
• takeWhile():EventStream
• toList():Future
• toSet():Future
• throttle():Reactable
• where():Future

Properties:

• first:Future
• last:Future
• length:Future
• single:Future
• isEmpty:Future