Improvements to styles(), js(), css()

`styles()` now has an optional argument which brings it in line with `scripts()` behavior of displaying the styles of each group name.

`js()` and `css()` now uses regex to detect protocol-relative URLs.
Example: 

    <script src="//ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
Merge pull request #5 from marsd/patch-5

Improvements to addStyle(), styles(), js(), css()