Require a currentDate property for DateSchedulers

This way, a TestScheduler can provide its own interpretation of the
“current date” for operators like Signal.interval.
Merge pull request #1515 from ReactiveCocoa/date-scheduler-current-date

Require a currentDate property for DateSchedulers
Merge remote-tracking branch 'origin/swift-development' into signal-tests
Test and update docs for Signal.buffer