add a few features kind of like async's forEachLimit but for what.js

PromiseStack.js

// example:
//
//    var promises = new PromiseStack();
//
//    for (var i = 0; i < 10; i++) promises.push(/* a promise */, /* args... */);
//
//    promises.limit(
//        4,
//        function () {
//            console.log('success');
//        },
//        function (error) {
//            console.log(error.message);
//        }
//    );
function PromiseStack () {
  this._calls = [];
}
PromiseStack.prototype = {
	push: function (func) {
		var args = Array.prototype.slice.call(arguments).slice(1);
		this._calls.push([func, args]);
	},
	series: function (callback, error) {
		var firstCall = this._calls.shift(),
			deferred = firstCall[0].apply(null, firstCall[1]);

		this._calls.forEach(function (call) {
			deferred = deferred.then(function () {
				return call[0].apply(null, call[1]);
			});
		});

		deferred.then(callback, error);

		return deferred;
	},
	limit: function (limit, callback, errorCallback) {
		var calls = this._calls,
			errored = false;
		
		function error (message) {
			errorCallback(errored = message);
		}

		function next () {
			if (errored) return;

			var call = calls.shift();
			call[0].apply(null, call[1]).then(calls.length ? next : callback, error);
		}

		for (var i = 0; i < limit; i++) next();
	}
};