MethodInvocation is missing a getThis() (or equivalent) method

The current (r896125) revision of MethodInvocation does not expose a method for accessing the object on which the method is being invoked.

In the AOP world, I think this is the JoinPoint.getThis() method.

In any case, such a method is necessary to test permissions at the instance level (e.g.: "lightsaber:wield:blue", where "blue" is obtained from the instance on which the "wield()" method of the "Lightsaber" class is called.)