Added support for User Switching plugin

I'm developing quite a large multisite project right now and for that I needed to view debug information when switching to another user (http://wordpress.org/extend/plugins/user-switching/). By default, Debug Bar only allows super admins to view debug information, so I added a version of `is_super_admin()` that also authenticates the auth cookie of the original user and if that user is a super admin, then debug information can be viewed as well.

I tested it only in v0.8, because the current 0.9-alpha didn't work for me. Would be great if this could be added.