Populate 'redirect_to' param on sign out

This will allow us to redirect to the app where the "sign out" link was
clicked. This caused no end to the confusion for internal testing when
we were juggling multiple logins.
Add flash notification when signing out

Let the user know that they've been logged out of the application.
Merge pull request #2 from RadiusNetworks/signout-redirect

Redirect back to the app when signing out