Add ARIA attributes to the site.

This make the next changes:

1. Add titles to the <a> tags
2. Add alt attributes to <img> tags
3. Add the role="navigation" to <nav> tag
4. Delete the css rule "color" on the #navigation
5. Change the css rule for #slideshow
6. Add role="footer" to the <footer> tag
7. Add role="main" to #content div
8. Add role="listitem" to #slivers div