SignalProducer.takeUntil: added overload with trigger as Signal
SignalProducer.takeUntil: added overload with trigger as Signal

This also introduces a `lift` overload that allows us to implement similar overloads to other operators if we need to.
Note that I haven't added any tests since the operator is written in terms of `lift` and `Signal.takeUntil` which **is** tested.

yonekawa/ReactiveCocoa,paulyoung/ReactiveCocoa,paulyoung/ReactiveCocoa,tzongw/ReactiveCocoa,Farteen/ReactiveCocoa,calebd/ReactiveCocoa,smilypeda/ReactiveCocoa,ohwutup/ReactiveCocoa,kevin-zqw/ReactiveCocoa,ikesyo/ReactiveCocoa,smilypeda/ReactiveCocoa,Juraldinio/ReactiveCocoa,sandyway/ReactiveCocoa,yonekawa/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,tonyarnold/ReactiveCocoa,walkingsmarts/ReactiveCocoa,llb1119/test,victorlin/ReactiveCocoa,llb1119/test,walkingsmarts/ReactiveCocoa,kevin-zqw/ReactiveCocoa,natestedman/ReactiveCocoa,llb1119/test,ohwutup/ReactiveCocoa,rpowelll/ReactiveCocoa,Farteen/ReactiveCocoa,calebd/ReactiveCocoa,kevin-zqw/ReactiveCocoa,calebd/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,tzongw/ReactiveCocoa,walkingsmarts/ReactiveCocoa,victorlin/ReactiveCocoa,tonyarnold/ReactiveCocoa,sandyway/ReactiveCocoa,rpowelll/ReactiveCocoa,tzongw/ReactiveCocoa,Farteen/ReactiveCocoa,ohwutup/ReactiveCocoa,natestedman/ReactiveCocoa,tonyarnold/ReactiveCocoa,rpowelll/ReactiveCocoa,paulyoung/ReactiveCocoa,andersio/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,natestedman/ReactiveCocoa,andersio/ReactiveCocoa,victorlin/ReactiveCocoa,yonekawa/ReactiveCocoa,sandyway/ReactiveCocoa,Juraldinio/ReactiveCocoa,andersio/ReactiveCocoa,RuiAAPeres/ReactiveCocoa,ReactiveCocoa/ReactiveSwift,ikesyo/ReactiveCocoa,smilypeda/ReactiveCocoa,Juraldinio/ReactiveCocoa,RuiAAPeres/ReactiveCocoa