Implement 'administer CiviDiscount' permission
Created by: MegaphoneJon
There's a stub from pre-Civi 4.2 expressing the intent to add an 'administer CiviDiscount' permission. This PR implements that permission, so you don't have to grant "administer CiviCRM" to work with discounts. Existing installs will be unchanged, since "administer CiviCRM" will still get you all the access it used to.
I also added CiviDiscount to the Events and Contributions menus since the Administer menu requires "administer CiviCRM". I also removed the old stub, since it referenced the incorrect name for the hook that was eventually created.