Supporting json_key in read_gdp
Google recommends using json key insted of private_key (.p12 file) to connect Google Apps.
I think the read_gbd should support json_key.
So I suggest read_gdp definition
```python
def read_gbq(query, project_id=None, index_col=None, col_order=None,
             reauth=False, verbose=True, private_key=None, json_file=None, dialect='legacy'):
```
and insert  like under codes

'''
    import json
    PRIVATE_KEY = 'private_key'
    if json_file is not  None and private_key is None:
        with open(json_file, 'rb') as json_file:
            private_key =  json.loads(f.read())[PRIVATE_KEY]
``` 

It need thinking more about the error handling ....