Fixed #1

Giving users the ability to specify additional attributes for script tag as per our discussion on this [issue](https://github.com/Wixel/CodeIgniter-AssetLoader/issues/1).  We discussed following syntax to assign additional attributes:

    js[]  = ["src" => "js/vendor/modernizr-2.6.2.min.js", "attributes" => ["async" => true, "custom-attribute" => "custom-attribute-value"]];

But I found this syntax a bit difficult to write for developers. Also `.ini` files don't allow inserting `=>` without any quotes. So I came up with a new and easy syntax

    js[]  = ["src => js/vendor/modernizr-2.6.2.min.js", "async => true"]

I have tested `script` tags on my side and they seems to work fine. Please check on your side and let me know how it goes.


Thanks.