Added parallelEach

Added `parallelEach` function, sometimes we need to execute only one iterator in parallel, but current *parallel* related functions don't support this functionality. So I wrote my own.

parallelEach(arr, iterator, callback)
========================

##arguments
* arr - An array to iterate over.
* iterator(item, callback) - A function to apply to each item in the array.The iterator is passed a callback(err, result) it must call on completion with an error (which can be null) and an optional result value.
* callback(err, results) - An optional callback to run once all the functions have completed. This function gets a results array (or object) containing all the result arguments passed to the task callbacks.

Example:
```javascript
async.parallelEach([100, 50, 150], function(item, callback){
        setTimeout(function(){
            console.log(item); //It will print the result as the order of 50, 100, 150
            callback(null, item);
        }, item);
}, function(err, results) {
   //the results array will equal [100, 50, 150].
});
```