Add admin user

PR to add an admin user. 

As discussed, to create an admin run `rake admin:set email=me@example.com` to remove an admin run `rake admin:remove email=me@example.com`. The rake task deals nicely with lack of email param, or missing user for a provided email.

This removes "Samples" from the navbar and adds a `before_filter` to the organisms_controller to check for admin status.