Sponsors

[Pivotal Tracker Story](https://www.pivotaltracker.com/story/show/64723776)

*Added `Sponsors` static page
*Added `Sponsors` sidebar
*Adjusted layouts for `Sponsors` sidebar to appear on all pages except front page and `Users`
*Created cucumber tests in features/sponsors.feature
*Created rspec tests in spec/views/layouts/sponsors.html.erb_spec.rb