The start of a major SASS refactor, plus minor changes:

- Renames `external` to `components`.
- Changes responsive nav menu from a JQuery one to the slimmer approach at http://responsive-nav.com.
- Adds header icon support.
- Header layout adjusted as per suggestions in #16
- Images now resized using CSS3 `calc`, so we can pad `main` and still have images stretch to the side of the screen.
- Deletes theme options, as we will not be using custom colours anymore
- Changes next posts link to paginated navigation
- Simplifies default post format

The SASS is being rewritten as per #17: now we have only two files, `reset.scss` and `style.scss`, generating the main layout. The colour scheme will be defined in a third file.