issubdtype(<categorical>, np.bool_) raises error
``` python
In[14]: import pandas as pd
Backend Qt4Agg is interactive backend. Turning interactive mode on.
In[15]: import numpy as np
In[16]: s = pd.Series([1,2,3,1,2,3]).astype("category")
In[17]: s
Out[17]: 
0    1
1    2
2    3
3    1
4    2
5    3
dtype: category
Categories (3, int64): [1 < 2 < 3]
In[18]: np.issubdtype(s.dtype, np.bool_)
Traceback (most recent call last):
  File "c:\data\external\ipython\IPython\core\interactiveshell.py", line 3032, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-18-607a91e2a828>", line 1, in <module>
    np.issubdtype(s.dtype, np.bool_)
  File "C:\portabel\miniconda\envs\ipython\lib\site-packages\numpy\core\numerictypes.py", line 763, in issubdtype
    return issubclass(dtype(arg1).type, arg2)
TypeError: data type not understood
```

This is a problem in https://github.com/pydata/patsy/pull/47

Not sure if there is an easy way to get numpy to understand this (I've absolutely no numpy foo :-/ ). If not this means that every patsy/statsmodels method which does dtype magic has to guard against the category dtype :-/
