Add footer to MyUSA

For non-Javascript clients, the footer will flow on the bottom
of the content, c'est la vie.

For Javascript clients,
- When the content doesn't fill the whole page, the footer will
  be pinned to the bottom of the page
- When the content fills the page, the footer will scroll with
  the content.

Update links to use Rails `link_to`
Make header fullscreen like footer

The line on the header should be fullscreen like the footer.

Update each of the headers in turn, and the css to match, such
that the header is fullscreen like the footer.  The content
is still inside the container.
Imports tests from #166

We'll need new tests for the marketing and dashboard pages,
but for now this implements basic tests.
Update MyUSA button and nav pils

The MyUSA button was just using btn-primary and not the actual
MyUSA button specified in the wireframes.  Having consistency
in our social buttons is important.

Create new `nav-4` and `nav-3` classes for the headers so that
they don't move around when hovered.  The problem was that
hovering made them bold, which changed their positioning.  Using
static % positioning fixes this optical challenge.

Revert profile link until the profile implementation is merged.
Merge pull request #174 from 18F/feature/footer

Add footer, improve MyUSA button, clean up CSS/HTML