[ADD] show development and difficulty fields of the aceptability.criteria
model at the user.story form view. Images:
- us form view
  https://docs.google.com/a/vauxoo.com/file/d/0B1UUUA6bRx8-U2hhd3dQTTh0YlE/edit?usp=drivesdk
- us/ac form view
  https://docs.google.com/a/vauxoo.com/file/d/0B1UUUA6bRx8-YzdkeXNxNWszUjA/edit?usp=drivesdk