Design changes: front page now display a third column for top actvities.

Same for activities page.
Background color changed to white
Size of the text went bigger

new file:   app/assets/images/Fly_Boarding-0.jpg
new file:   app/assets/images/orange-back-light.jpg
modified:   app/assets/stylesheets/list.scss
modified:   app/assets/stylesheets/nav.scss
modified:   app/assets/stylesheets/search.scss
modified:   app/assets/stylesheets/variables.scss
modified:   app/views/activities/_activities_sorted_list.html.erb
modified:   app/views/activities/_sentence_search.html.erb
modified:   app/views/activities/index.html.erb
modified:   app/views/activities/show.html.erb
modified:   app/views/categories/show.html.erb
Merge branch 'design'