Fixed ActionDisposable not disposing correctly, as the underlying value was never reset to nil.
Simplify Scheduler definitions
Add currentScheduler global property
Tighten up Atomic interface
Remove “unannotated” from Scheduler docs

This only made sense in the context of
0e9ab8961a1ee5521dc7e4ce529f3e98d1f80565.
Merge pull request #13 from jspahrsummers/concurrency-improvements

Concurrency improvements
Merge branch 'master' into pr/11
Atomically nil out ActionDisposable._action
Merge pull request #11
Add scheduleAfter methods
Add Observable.interval
Add ignore() over Streams
Add Stream.scan
Add Stream.combinePrevious
Add nub() over Streams
Add AsyncSequence.doEvent

This isn’t appropriate for Observable, since side effects can be
“injected” with a new subscription, which should be basically the same
thing.
Fix definition of scan()
Add Stream.aggregate
Add Observable.delay
Add Observable.deliverOn