Added autoTriggerRateLimit, autoTriggerPauseUntilLoaded functionality…

[rexfordkelly]

… and autoTriggerLastFiredAt property to jScroll along with sensible defaults.

I ran across the jScroll library in the wild... I noticed that particular implementation was a bit broken, from a low bandwidth users perspective, as the page kept auto triggering before any of the images even loaded... I'm sure this is the fault of the site developers, for not styling their HTML so their images had sensible pre-defined dimensions... but I thought I would contribute a simple patch to account for that.