This commit addresses Issues #2, #3 and #6. See extended commit message for further details.

Resolves Issue #2:

* I have removed all references of 'jf::CurrentUser()', made $UserID mandatory, and have added an exception if the $UserID is not provided. This exception will add flexibility to PhpRbac's testability and error handling capabilities.
* I have updated all affected Unit Tests associated with these changes, while adding Unit Tests as needed

Resolves Issue #3:

* Along with the changes from Issue #2, I have removed the references to jb::run and added the 403 header along with a die which displays a 'Forbidden' message if the check in $rbac->enforce fails
* If the check in $rbac->enforce is successful, $rbac->enforce will return true for testing and error handling purposes

Resolves #6:

* UserID is now mandatory and will throw an exception if the parameter is empty