Update marketing page with images, styles

- Add retina images for MyUSA logo and MyUSA shield
- Update responsive design styles
- Add links for legal, faq
- Add images for partner logos
- Add styling to partner logos
Add applications to drop down menu

The menu was lonely, and this isn't accessible
from the main navigation header unless you're in the
dashboard.  So give people a way to go directly here.
Open "steps" buttons in new tab

After discussion with @dutchashell, we should open these in
a new window so people can come back to where they started
in the 3 step process.
Update rspec to match button text

UI updated to be more user friendly; tests needed to be updated
to match the changes.
Merge pull request #267 from 18F/feature/marketing-improvements

Feature/marketing improvements