transfering DataFrame to JSON using dictionary style with Type Error
#### Code Sample, a copy-pastable example if possible

I want to transfer the follwing dataframe to JSON using a dictionary style. 

```
   weightedRisk  riskAsset  safetyAsset  weightedAsset  riskRank
0      0.029839   0.029839     0.220161           0.25         2
1      0.053840   0.053840     0.446160           0.50         3
2      0.015485   0.015485     0.234515           0.25         4
```
#### Expected Output

the expected ouput should be:

```
   "groupedWeightedRisk": {
        "safetyAsset": [
            0.2201611228,
            0.4461597981,
            0.2345148013
        ],
        "riskAsset": [
            0.0298388772,
            0.0538402019,
            0.0154851987
        ],
        "riskRank": [
            2,
            3,
            4
        ],
        "weightedRisk": [
            0.0298388772,
            0.0538402019,
            0.0154851987
        ],
        "weightedAsset": [
            0.25,
            0.5,
            0.25
        ]
    }
```

But in pandas, all of the **to_json** method makes the ouput contains the index.

So.I  use **to_dict(orient='list")** and then using  **json.dumps** to serialize the data.

this method works well in most of the time. However , in the pervious case, the 
**json.dumps** raise an error saying:
**TypeError: 2 is not JSON serializable**

then I checked the dataframe and locate the problem.

```
type(groupedWeightedRisk['riskRank'][0])
<class 'numpy.int64'>
```

one of the series is made up by **numpy.int64**, and this data type could not be JSON serializable directly.

Finally, I solve this problem using a function with a stupid way,that I serialise the data columns one by one.

```
def dataFrameToDict(df,indexMethod=None):
    result={}
    for i in df.columns:
        result[i]=json.loads(df[i].to_json(orient='records'))
    if indexMethod is not None:
        if df.index.name is not None:
            indexName=df.index.name
        else:
            indexName='index'

        if type(df.index) == pd.tseries.index.DatetimeIndex:
            result[indexName]=list(df.index.strftime("%Y-%m-%d"))
        else:
            result[indexName] = list(df.index)
    return result
```

So, is there any possibility that pandas could add a method in **to_json** that could provide the a JSON data using dictionary style? 

Thanks a lot
