df.to_json segfaults with categorical column types
This code from the Categorical tutorial:

``` python
df = pd.DataFrame({"A":["a","b","c","a"]})
df["B"] = df["A"].astype('category')
```

Will segfault when `df.to_json` is called.  Tested on:

``` bash
Python 3.4.3 |Anaconda 2.3.0 (x86_64)| (default, Mar  6 2015, 12:07:41)
[GCC 4.2.1 (Apple Inc. build 5577)] on darwin
$ conda list  | grep pandas
pandas                    0.16.2               np19py34_0
```
