Limit concurrency of a function returning a promise.
var throttle =var throttledFn =
throttleFn functions like
anyFn, but throttled to a concurrency of
concurrency, where concurency is a number equal or higher than one.
throttledFnreturns a promise.
anyFnmust return a promise. If it does not, the promise returned by
throttledFnwill be rejected.
f-throttle is released under the MIT License.
Copyright (c) 2013 Meryn Stol