Add conditional helper methods

[cjmellor]

This PR adds some helper methods to the Model to run state change based on if a conditional is met or not.

$approval->approveIf(true);
$approval->rejectIf(false);
$approval->postponeIf(true);

$approval->approveUnless(false);
$approval->rejectUnless(true);
$approval->postponeUnless(false);