title: Filter Pandas DataFrame by comparing columns in a row
body: <p>Each row in my DataFrame has two date columns. How can I filter out the rows in which 'Date A' is after 'Date B'?</p>
  
  <p>Example:  </p>
  
  <pre><code>symbol |    reports_at       |    as_of        |      signal   
  ...   
  A     | 2012-02-15T21:00:00Z |  2012-02-01T12:00:00Z|   65.20464367   
  ...
  </code></pre>
  
  <p>This row should be deleted from the DataFrame because the date in the 'reports_at' column occurs after the date in the 'as_of' column</p>