title: Convert epoch time to formatted date string in pandas dataframe
body: <p>I have a DataFrame with epoch timestamp and I want to create a new column with a formatted date string.</p>
  
  <pre><code>index   timestamp
  0       1456407930
  1       1456407945
  2       1456407961
  3       1456407977
  4       1456407992
  5       1456408008
  6       1456408024
  7       1456408040
  8       1456408055
  9       1456408071
  10      1456408087
  11      1456408102
  </code></pre>
  
  <p>First, I successfully convert the timestamp to datetime format using </p>
  
  <pre><code>data['date_num'] = mdate.epoch2num(data['timestamp'])
  </code></pre>
  
  <p>But I didn't find a function to get a new column with a string formatted date <code>(such as “%Y-%m-%d”)</code>.</p>
  
  <p>I would appreciate any ideas?
  Patricio</p>