pandas.DataFrame.to_json gives OverflowError when attempting to serialize datetime.time objects
`datetime.time` objects cause `to_json` to fail, presumably because they are attempting to be converted in the same way as `datetime.datetime` objects. 

For example:

```
>>> pd.DataFrame(data=dict(a = [datetime.time()])).to_json()
...
OverflowError: Maximum recursion level reached
```

In contrast: 

```
 >>> pd.DataFrame(data=dict(a = [datetime.datetime.now()])).to_json()
'{"a":{"0":1379498870147}}'
```

Tested on 0.12 and HEAD (b039c1d33722bfdac8c665f9b1f3af44782e0e58)
