Add throtteling to parallel function execution

[fbuchinger]

underscore.js has a quite useful _.throttle function that prevents too frequent function invocation.
http://documentcloud.github.com/underscore/#throttle

This could also be a useful parameter for jsdeferred's parallel function (only execute a maximum of n parallel functions concurrently - queue the execution of the rest meanwhile)

[cho45]

I agree that _.throttle is useful.

But I can't image an use case of the parameter in parallel function.