param_sets method not working with SerialCompositeModel

It looks like the embedded models don't "pass" the parameters to the SerialCompositeModel instance:

The following code:

```
x = np.arange(5)
off = apm.models.Shift(-3.2)
poly1 = apm.models.Polynomial1D(3)
serial_composite_model = apm.SerialCompositeModel([off, poly1], inmap=[['x'], ['y']], outmap=[['y'], ['z']])
print serial_composite_model.param_sets
```

produces the following error:

```
ERROR: AttributeError: 'SerialCompositeModel' object has no attribute 'offsets' [astropy.modeling.core]
Traceback (most recent call last):
  File "test_mod.py", line 33, in <module>
    print serial_composite_model.param_sets
  File "/Library/Frameworks/Python.framework/Versions/Current/lib/python2.7/site-packages/astropy/modeling/core.py", line 222, in param_sets
    parameters = [getattr(self, attr) for attr in self.param_names]
AttributeError: 'SerialCompositeModel' object has no attribute 'offsets'
```