Formatting, cleaned up responsiveness

main.css
- fixed path issue on list items with class .navigation-main-list.
- added responsive design for the main list items to take up 100% width
and use the proper backgrounds.

index.html
- added viewport back in, looked like this may have been taken out
accidently, noticed issues with width when viewing on phones.
- added backgrounds for each list item instead of sprite, each li will
now take up 100% width.
- cleaned up formatting