Modified ACL::required(), added ACL::redirect()

#### The ACL 2.0

##### The second step for improving Gleez_ACL class
The beginning of this work was initiated in #61 and in #124

##### Changelog
* Cleanup code
* Amended PHPDoc docblocks
* Added `ACL::redirect()` method
* Added more constants for usability
* Changed to public mode some methods
* Modified `ACL::required()` method
* Refactoring some variables
* More minor changes
Refactoring according to https://github.com/gleez/cms/issues/24#issuecomment-14318916

##### Also
See comments & diff