Add administration capability

I've added the ability to edit member profiles as an administrator. Features:

* Admin login (link in footer) uses theodi.org google apps accounts for authentication. There is no ability to register new admins within the app. Any odi google account can administer, currently.
* Admins cannot change passwords. Users must do that themselves using the 'forgot password' actions.
* As well as the standard editing interface, admins can access a rails_admin panel for direct model editing. As this is not the ideal method, this is not linked and must be accessed directly. More for tech team use than anything else.

I'm happy that this is good to go, but we won't merge at this point. Perhaps @pezholio can test on Monday, and merge in when he's happy.