Remove -errors and -executing overrides

Also unify their main thread delivery.
Ensure that errors/executing complete on the main thread
Triple slashes for dem comments
Merge pull request #910 from ReactiveCocoa/replace-commands-with-actions

Replace RACCommand with RACAction
Merge pull request #915 from ReactiveCocoa/deprecate-sequences

Deprecate RACSequence and RACStream
Merge remote-tracking branch 'origin/3.0-development' into subscriber-disposal