Fixing navigation bug and markup

I've fixed the bug where the navigation goes the same color as the background on hover. I also cleaned up and made the markup valid and semantic for both sets of navigation in the header and the footer navigation.

One note: I noticed some Sass files, but wasn't sure if they were still being used. I made my changes in the CSS file, screen.css as I didn't see any of the styles I was looking for in the Sass files. If I'm wrong, I'm happy to go back and move the changes to a different file.

Thanks!