add is_admin attribute to users

app/views/admin/users/_roles.html.haml

@@ -0,0 +1,12 @@
+- if current_user == user
+  You cannot modify your own role!
+  %br
+  %button{:class=> "btn btn-danger", "data-dismiss"=> "modal", "aria-hidden"=>"true"}
+    Cancel
+- else
+  = "Give #{user.name} (#{user.email}) the following roles:"
+  = semantic_form_for(user, :url => admin_user_path(user), :method => :put) do |f|
+    = f.input :roles, :label => false, collection: @roles
+    %button{:class=> "btn btn-danger", "data-dismiss"=> "modal", "aria-hidden"=>"true"}
+      Cancel
+    = f.action :submit, :as => :button, :button_html => {:value => "Save", :class => "btn btn-primary"}