allow registration and signin/out on the API

app/controllers/registrations_controller.rb

@@ -1,4 +1,5 @@
 class RegistrationsController < Devise::RegistrationsController
+  respond_to :json
 
   def edit
     @twitter_auth  = current_member.auth('twitter')

app/controllers/sessions_controller.rb

@@ -1,4 +1,6 @@
 class SessionsController < Devise::SessionsController
+  respond_to :json
+
   def create
     super do |resource|
       if Crop.pending_approval.present? && current_member.has_role?(:crop_wrangler)