title: Pandas dataframe to json list format
body: <p>I have large pandas tabular dataframe to convert into JSON.
  The standard .to_json() functions does not make a compact format for JSON.
  How to get JSON output forma like this,  using pandas only ?</p>
  
  <pre><code>{"index": [ 0, 1 ,3 ],
   "col1": [ "250", "1" ,"3" ],
   "col2": [ "250", "1" ,"3" ]
  }
  </code></pre>
  
  <p>This is a much compact format form of JSON for tabular data.
  (I can do a loop over the rows.... but)</p>