Display message about oc-id when users sign out.

:fork_and_knife: 

Signing out of Supermarket does not sign users out of oc-id, which is the
expected behavior. However, it is not clear when signing out that this is the
behavior. This commit displays a message when a user is signed out letting them
know they need to go to id.opscode.com to signout completely.

Closes #682.

What it looks like:

![screen shot 2014-07-31 at 9 50 55 am](https://cloud.githubusercontent.com/assets/928367/3765404/422b664a-18bb-11e4-8161-8dd4d8a63194.png)

This has been added in addition to the flash message because the flash message disappears after some time. This is more permanent and prominent.