Created a Forbidden Variables names checker, to flag $_REQUEST

This is a solution to a enhancement request at https://github.com/Automattic/vip-scanner/issues/99
Tests included.
Changed the way $_REQUEST variable is flagged

Instead of have a dedicated class to check for forbidden variables, it is now using the vip-scanner/checks/VIPRestrictedPatternsCheck.php to check for the $_REQUEST variable usage. It is flagged as a Blocker.
Also, the tests have been changed.
Merge pull request #140 from vaurdan/flag_forbidden_vars

Flag $_REQUEST superglobals