Partner App Logout

Supports a redirect after logout.  By passing "continue=<url>" as a logout
parameter, an app can redirect the user back to any sub-url of their site.  In
order to be allowed, the redirect URL must be a hostname that ends with the
same string as an application's URL, for an application approved by the user.
Merge branch 'devel' into feature/api-logout

Conflicts:
	app/controllers/application_controller.rb
Merge branch 'devel' into feature/api-logout
Update Aglio Docs
Merge pull request #241 from 18F/feature/api-logout

Partner App Logout
Merge branch 'devel' into feature/manage-approved-apps