BUG: Series/Dataframe  to_json not returning miliseconds
According to the documentation for pandas 0.12

> date_format : type of date conversion (epoch = epoch milliseconds, iso = ISO8601)
>     default is epoch

The json exporting function seems to be returning the epoch _nanoseconds_ rather than the _milliseconds_ as suggested by the documentation.

``` python

s = pd.Series(1, index=pd.date_range('2013-07-25', periods=2, ))
s.index.astype('int64')
>> array([1374710400000000000, 1374796800000000000])

s.to_json(orient='split', date_format='epoch')
>> '{"name":null,"index":[1374710400000000000,1374796800000000000],"data":[1,1]}'
```
