title: Compare columns of Pandas dataframe for equality to produce True/False, even NaNs
body: <p>I have two columns in a pandas dataframe that are supposed to be identical. Each column has many NaN values. I would like to compare the columns, producing a 3rd column containing True / False values; <em>True</em> when the columns match, <em>False</em> when they do not.</p>
  <p>This is what I have tried:</p>
  <pre><code>df['new_column'] = (df['column_one'] == df['column_two'])
  </code></pre>
  <p>The above works for the numbers, but not the NaN values.</p>
  <p>I know I could replace the NaNs with a value that doesn't make sense to be in each row (for my data this could be -9999), and then remove it later when I'm ready to echo out the comparison results, however I was wondering if there was a more pythonic method I was overlooking.</p>