Change the style and script paths from absolute to use use the asset helper

* Updated styles and scripts on layout to use asset helper.  This will allow the theme to work if the blog root is in a subdirectory.  i.e. http://my-ghost-blog.com/subfolder/

* Made external resources load over same protocol

Example...

```
<link rel="stylesheet" type="text/css" href="{{asset "assets/css/normalize.css"}}" />
<script type="text/javascript" src="{{asset "js/index.js"}}"></script>
```