FEATURE: Add rejection message on rejected IncomingEmail

FIX: Better RateLimit description in rejected IncomingEmail
FEATURE: Send email when hitting a rate limit

db/migrate/20160303234317_add_rejection_message_to_incoming_email.rb

@@ -0,0 +1,5 @@
+class AddRejectionMessageToIncomingEmail < ActiveRecord::Migration
+  def change
+    add_column :incoming_emails, :rejection_message, :text
+  end
+end