Cleaning up the site header.

It now looks like the other ATN Rails applications.

* Removed unused links.
  * Change password
  * Edit Users
  * Sign up
* Placed the header in a container.
* Added `atni-logo.png`.
* Added sitewide styling.
Merge pull request #14 from ATNI/cleanup-header

Cleaning up the site header.
Merge branch 'master' into extracting-user-methods

Conflicts:
	app/views/layouts/_header.html.erb

Resolved conflict by accepting new styling changes from master branch.