Introducing a $_REQUEST/POST/GET wrapper

This new wrapper ensures types are correct and accessed parameters are
actually set (with custom default fallbacks).

The wrapper is available in the global $INPUT variable. It accesses
$_REQUEST by default. If POST or GET is required, the post and get
members can be used:

$INPUT->int('foo',false); // access $_REQUEST['foo'], default false
$INPUT->post->int('foo'); // access $_POST['foo'], default 0
$INPUT->get->int('foo'); // access $_GET['foo'], default 0

The codebase still needs to be updated to make use of this.