Elena

• Subscription disposables now only create strong references to sinks until being disposed or sequence terminates. #573
• Introduces SharedSequence and makes Driver just a specialization of SharedSequence.
  That means Driver is now just one specific SharedSequence and it is now possible to easily create new concepts that have another compile time guarantees in a couple of lines of code.
  E.g. choosing a background scheduler on which elements are delivered, or choosing share as a sharing strategy instead of shareReplayLatestWhileConnected.
• Moves Reactive struct and ReactiveCompatible from RxCocoa to RxSwift to enable third party consumers to remove RxCocoa dependency.
• Add rx. extensions on Types.
• Moves UIImagePickerViewController and CLLocationManager out of RxCocoa to RxExample project because of App Store submissions issues
  on iOS 10.
• Adds sentMessage got its equivalent sequence methodInvoked that produces elements after method is invoked (vs before method is invoked).
• Deprecates observe method on DelegateProxy in favor of sentMessage.
• Adds simetric methodInvoked method on DelegateProxy that enables observing after method is invoked.
• Moves all delegate extensions from using sentMessage to using methodInvoked (that fixes some problem with editing data sources)
• Fixes problem with RxTableViewDataSourceProxy source enabling editing of table view cells (swipe on delete) even if there weren't
  any observers or forwardToDelegate wasn't implementing UITableViewDataSource.tableView(_:commit:forRowAt:). #907
• Makes DelegateProxy open. #884
• Deprecates extensions that were polluting Swift collection namespaces and moves them to static functions on Observable
  • Observable.combineLatest
  • Observable.zip
  • Observable.concat
  • Observable.catchError (sequence version)
  • Observable.amb
• Deprecates extensions that were polluting Swift collection namespaces and moves them to static functions on Driver
  • Driver.combineLatest
  • Driver.zip
  • Driver.concat
  • Driver.catchError (sequence version)
  • Driver.amb
• Update Getting Started document, section on creating an observable that performs work to Swift 3.0.
• Removes stale installation instructions.