Add a prototype for setting up callbacks on a method from a class object

Here you go, this works for adding callbacks at the class- and instance- level.  e.g.

```python
class Car(object):
    def __init__(self, name, speed):
        self.name = name
        self.speed = speed

    @supports_callbacks
    def get_speed(self):
        print "getting speed"
        return self.speed

# @pre_callback(Car.speed, takes_target_args=True)
def mycallback1(self):
    print "how's my driving?"

# @post_callback(Car.speed, takes_target_results=True)
def mycallback2(speed):
    if speed < 55:
        print "that's pretty slow"
    else:
        print "nice and fast"

# add to all cars:
Car.get_speed.add_post_callback(mycallback2, takes_target_result=True)

ferrari = Car('ferrari', 180)
truck = Car('truck', 50)

# add to just truck:
truck.get_speed.add_pre_callback(mycallback1, takes_target_args=True)

print "calling ferrari:"
ferrari.get_speed()
print

print "calling truck:"
truck.get_speed()
```

prints:
```
calling ferrari:
getting speed
nice and fast

calling truck:
how's my driving?
getting speed
that's pretty slow
```

It's not perfect, but perhaps that will get you unstuck.  If you get the tests running I'll add some to test this case.

I agree that updating every instance of a class is a bad idea.  It wouldn't catch the case when you make new instances of the class either, which would lead to some really confusing behavior.