to_json should validate its arguments
```
In [67]: df
Out[67]:
          counter
created
05-16-13        3
05-17-13        1
05-18-13        1

In [68]: print df.to_json('asdfasdf')
None

In [69]: print df.to_dict('series')
{'counter': created
05-16-13    3
05-17-13    1
05-18-13    1
Name: counter, dtype: int64}

In [70]: print df.counter.to_json('asdfasdf')
None

In [71]: print df.counter.to_json('index')
None
```

The "default" value of `'index'` (whose default is actually `None`) doesn't do what it says.
