Major rewrite of code

Major rewrite of code
- Made theme responsive down to a minimum width of 600px
- Fixed layout of sidebars and main content, when sidebar is inactive
the main content will revert to being centred
- Fixed style.css to include the base wordpress added classes
- Fixed the header and footer document to include the stylesheets and
scripts in wp_head or wp_footer, (also fixed this in functions.php to
register and enqueue)
- Created the slider for the front page
- Replaced some images with placeholder images
- Fixed some errors in style.css, including making everything sized
using border-box by default in CSS