adding functionality, documentation for banner image

Should contain all necessary edits to implement the banner image. A few notes:

* Subtitle is optional, displaying banner on all pages or just index page supported.
* Banner plays well with mobile by downsizing text 
* I really wanted to avoid it, but this required a single line of javascript. Problem is, the Navbar floats over the `<body>`, and as of right now the body is just default padded on top by a default 70px regardless. But because different bootswatch themes have different height navbars, to get the banner to line up with the edge of the navbar requires logic, or there will be overlap or whitespace. Code contained in the bodypadding.js, uses jQuery. 

I tested this implementation with a clean site and a variety of different config combinations, but let me know if additional changes are necessary. Should be ready to go.