Json import sorting
Noticed that if export a df to json file and then reload the data, ordering is not kept.

``` python
import pandas as pd

df.to_json('test.json')
newdf = pd.read_json('test.json')

print df.index
```

gives:

``` python
1
10
100
....
```

instead of 

``` python
1
2
3
....
```

it suffices to call `newdf = pd.read_json('test.json').sort()` to have the what I'd expect.

Is this intended behaviour or bug?
