df.to_stata doesn't handle boolean columns
I'd expected this to generate a row of 1's and 0's in the dta file...

```
DataFrame([True, False]).to_stata('test.dta')
```

But it generates the following error:

sbox/lib/python2.7/site-packages/pandas/io/stata.pyc in _dtype_to_stata_type(dtype)
    884     else:  # pragma : no cover
    885         raise ValueError("Data type %s not currently understood. "
--> 886                          "Please report an error to the developers." % dtype)
    887 
    888 

ValueError: Data type bool not currently understood. Please report an error to the developers.
