[add] added maximum client and demo client protection

- default demo clients can no longer be modified/deleted when in demo mode
 - increased default demo reset to 60 minutes

config/application.rb

@@ -58,5 +58,6 @@ module RynDNS
       puts "WARNING: Please modify api_key -> #{API_KEY_PREFIX}.########.############"
     end
     API_KEY_HEADER = ENV['RAILS_API_KEY_HEADER'] || 'X-API-Key'
+    MAX_CLIENTS = ENV['RAILS_MAX_CLIENTS'] || 250
   end
 end