add make a user an admin, closing (#26)

app/controllers/users_controller.rb
+  def adminify
app/models/user.rb
+  def adminify!
app/views/users/index.html.haml
+      %th= 'Admin?'
       Don't check for admin, they are the only people who can hit this page
app/views/users/show.html.haml
+  %td= link_to 'Make Admin', adminify_useradmin_path(@user), class: 'btn btn-default'
config/routes.rb
+      get 'adminify'
       get 'impersonate'
       get 'stop_impersonating'
     end
spec/features/make_admin_spec.rb
+  describe 'as an admin' do
+    it 'I can make another user an admin' do