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
Hi Rafael,

The screen reader tool for Mac OS “VoiceoOver", reads the links as follow: “Link Plataformatec“ 

 And when we remain in the link for a few seconds, the tool reads the title attribute as a "help tag”, as follow:
“You are currently on a link, inside of a HTML content. To click this link, press control-option-space. The help tag is: Projects Inception, Design and Software Development “

The goal of the link title attribute is to help users with disabilities to provide additional information and predict what will happen if they follow a link. So, a link title attribute may be superfluous if it simply repeats the same text as is already shown in the anchor. (http://www.w3.org/TR/WCAG20-TECHS/H33.html)