Updates to CSS, reorganisation of project files, minor JS changes

I've updated the CSS file to make a few improvements. Added commenting to the file, removed all the hacks, improved the appearance in older browsers (like IE < 9 with no support for box-shadow), and improved the use of CSS3 properties.

Reorganised the project following the addition of a dedicated "demo" page.

Minor changes to the the JS so that it is more consistent (spacing between operators, use of braces, etc). But also added a few attributes to the close button.