Introducing breaking points and media queries

Using 3 different css files at the moment:
-mobile.css is the default one and is loaded first
-600.css is used when the viewport's width is at least 600px
-900.css is used when the viewport's width is at least 900px

Media queries at 600px and 900px might be changed to other values later.
Additional breaking points may also be added.

For testing:
-mobile.css applies a yellow BG to <body>
-600.css applies a green BG to <body>
-900.css applies a white (target colour) BG to <body>

When development's over, all CSS files will be combined into one to
minimise the number of HTTP requests required