title: add array into pandas dataframe
body: <p>I have a pandas dataframe called A with one column called "a":</p>
  
  <ul>
  <li>date        a</li>
  <li>2016-01-19  3</li>
  <li>2016-01-20: 1 </li>
  <li>2016-01-21: 2</li>
  </ul>
  
  <p>I have one array that looks like: [4,3,2]. I want to insert this array into the dataframe and give the new column the name b. How do I do that?</p>
  
  <p>Expected output:</p>
  
  <ul>
  <li>date        a  b</li>
  <li>2016-01-19  3  4</li>
  <li>2016-01-20: 1  3</li>
  <li>2016-01-21: 2  2</li>
  </ul>