Pass context and args as arguments in _.debounce

I was playing around with the `setTimeout` arguments to _.debounce and in addition to killing a few lines the [perf win](http://jsperf.com/smaller-debounce) on FF and Safari is actually pretty significant (no difference on Chrome). Mostly I'm interested in getting rid of the `context = args = null` lines so we don't have to worry about memory leaks.