title: add a 'now' timestamp column to a pandas df
body: <p>I have the following code:</p>
  
  <pre><code>s1 = pd.DataFrame(np.random.uniform(-1,1,size=10))
  s2 = pd.DataFrame(np.random.normal(-1,1, size=10))
  s3 = pd.concat([s1, s2], axis=1)
  s3.columns = ['s1','s2']
  </code></pre>
  
  <p>Which generates a DF that looks like this:</p>
  
  <pre><code>    s1          s2
  0   -0.841204   -1.857014
  1    0.961539   -1.417853
  2    0.382173   -1.332674
  3   -0.535656   -2.226776
  4   -0.854898   -0.644856
  5   -0.538241   -2.178466
  6   -0.761268   -0.662137
  7    0.935139    0.475334
  8   -0.622293   -0.612169
  9    0.872111   -0.880220
  </code></pre>
  
  <p>How can I add a column (or replace the index 0-9), by a timestamp with the now time? The np array will not always have size 10</p>