Add -fireDate to match a similar property on NSTimer

MSWeakTimer is a great replacement for NSTimer, however I did stumble on one scenario which was not carried over from NSTimer.

Namely, when you create a repeating timer, it sometimes makes sense to postpone the start of the timer to some specific moment, whether it be to match the displayed time of the device or some other reason. NSTimer offers -fireDate/-setFireDate for that purpose, so I brought an analogue to MSWeakTimer to replicate the effect.