title: Pandas: create json file from dataframe
body: <p>I have dataframe</p>
  
  <pre><code>ID, visiting
  111, 03.2015
  111, 07.2015
  111, 05.2016
  222, 12.2013
  222, 04.2016
  333, 02.2014
  333, 06.2015,
  333, 11.2015
  </code></pre>
  
  <p>I need to get file like this(I need to specify all month since december 2013 to june 2016)</p>
  
  <pre><code>{
    "111": {
  "2013-12": 0, 
  "2014-01": 0, 
  "2014-02": 0, 
  "2014-03": 0, 
  "2014-04": 0, 
  "2014-05": 0, 
  "2014-06": 0, 
  "2014-07": 0, 
  "2014-08": 0, 
  "2014-09": 0, 
  "2014-10": 0, 
  "2014-11": 0, 
  "2014-12": 0, 
  "2015-01": 0, 
  "2015-02": 0, 
  "2015-03": 1, 
  "2015-04": 0, 
  "2015-05": 0, 
  "2015-06": 0, 
  "2015-07": 1, 
  "2015-08": 0, 
  "2015-09": 0, 
  "2015-10": 0, 
  "2015-11": 0, 
  "2015-12": 0, 
  "2016-01": 0, 
  "2016-02": 0, 
  "2016-03": 0, 
  "2016-04": 0, 
  "2016-05": 1, 
  "2016-06": 0
  }, 
    "222": { ...
  }
  }
  </code></pre>
  
  <p>How can I get that from Pandas?</p>