title: Calculate an incremental mean using python pandas
body: <p>I'd like to generate a series that's the incremental mean of a timeseries.  Meaning that, starting from the first date (index 0), the mean stored in row x is the average of values [0:x]</p>
  
  <pre><code>data
  index   value   mean          formula
  0       4
  1       5
  2       6
  3       7       5.5           average(0-3)
  4       4       5.2           average(0-4)
  5       5       5.166666667   average(0-5)
  6       6       5.285714286   average(0-6)
  7       7       5.5           average(0-7)
  </code></pre>
  
  <p>I'm hoping there's a way to do this without looping to take advantage of pandas.</p>