Css revision & sass version

Hi there,

I thought I would submit this pull request for your review as I have made some edits that have reduced the CSS and cut out some redundancies, as well as made it a bit easier to drop into an existing project by targeting classes as opposed to generic HTML elements.

I've also converted the CSS over to Sass and the CSS file is generated from the new scss file.

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Here are some additional notes about my commit:

removed font-weight:400 on hover as it had a weird delay in some browsers that made it a choppy animation.

add new classes for slide nav so that the code can be more easily dropped into an existing project w/out having to worry about style conflicts.

changed up some of the padding / margins of the nav bar so that the icons are now included in the hover/hit area for links.

added box-sizing: border-box to the css and adjusted widths in response to this.

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Thanks you for considering this request.