title: Get the average in a dataframe up to each observation
body: <p>I want to get the average score by Name going up to the date.</p>
  
  <p>Here is an example dataframe</p>
  
  <pre><code>Date    Name  Score
  
  1/1/16  Bill   5
  1/1/16  Hank   5 
  1/1/16  Aaron  10
  1/2/16  Hank   15
  1/2/16  Bill   10
  1/3/16  Bill   20
  1/3/16  Aaron  20
  1/4/16  Aaron  20
  </code></pre>
  
  <p>How can I add this column?</p>
  
  <pre><code>Date    Name  Score Average_to_date
  
  1/1/16  Bill   5     NA
  1/1/16  Hank   5     NA
  1/1/16  Aaron  10    NA
  1/2/16  Hank   15    5
  1/2/16  Bill   10    5
  1/3/16  Bill   20    7.5
  1/3/16  Aaron  20    10
  1/4/16  Aaron  20    15
  </code></pre>