@nden Is this a bug or am I using `MatrixRotation` incorrectly?
```
In [1]: from astropy.models.rotations import MatrixRotation2D

In [2]: m = MatrixRotation2D(angle=42)
ERROR: AssertionError: Expected angle to be a number [astropy.models.rotations]
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-2-77a226a12d7e> in <module>()
----> 1 m = MatrixRotation2D(angle=42)

/Users/deil/Library/Python/2.7/lib/python/site-packages/astropy-0.3.dev3922-py2.7-macosx-10.8-x86_64.egg/astropy/models/rotations.pyc in __init__(self, rotmat, angle)
    173                                                                                 outdim=1, paramdim=1)
    174         else:
--> 175             self._validate_angle(angle)
    176             self._angle = Parameter('angle', np.deg2rad(angle), self, 1)
    177             super(MatrixRotation2D, self).__init__(parnames=[], ndim=1,

/Users/deil/Library/Python/2.7/lib/python/site-packages/astropy-0.3.dev3922-py2.7-macosx-10.8-x86_64.egg/astropy/models/rotations.pyc in _validate_angle(self, angle)
    199         a = np.asarray(angle)
    200         assert isinstance(a, numbers.Number), \
--> 201                     "Expected angle to be a number"
    202         assert a.ndim == 0, \
    203                     "Expected angle to be a number"

AssertionError: Expected angle to be a number
```