title: R: Keep number of open issues by date
body: <p>My data is just 2 columns -- when something started and day it started on
  I want to keep a running total of the number of things open. </p>
  
  <p>For example, here is my data: </p>
  
  <pre><code>Issue      Day Started  Day Stopped
  A          7-01-2013    Still Open
  B          7-04-2013    7-06-2013
  C          7-09-2013    7-10-2013
  D          7-09-2013    7-11-2013
  E          7-09-2013    Still Open
  F          7-11-2013    Still Open
  </code></pre>
  
  <p>This is what I would like my data to look like:</p>
  
  <pre><code>Date       Issues Open
  7-01-2013  1
  7-02-2013  1
  7-03-2013  1
  7-04-2013  2
  7-05-2013  2
  7-06-2013  1
  7-07-2013  1
  7-08-2013  1
  7-09-2013  4
  7-10-2013  3
  7-11-2013  3
  </code></pre>
  
  <p>Any tips on how to get this? Thanks!</p>