title: subtract value from previous row by group
body: <p>In R, let's say I have this data frame:</p>
  
  <pre><code>Data
  id      date        value
  2380    10/30/12    21.01
  2380    10/31/12    22.04
  2380    11/1/12     22.65
  2380    11/2/12     23.11
  20100   10/30/12    35.21
  20100   10/31/12    37.07
  20100   11/1/12     38.17
  20100   11/2/12     38.97
  20103   10/30/12    57.98
  20103   10/31/12    60.83 
  </code></pre>
  
  <p>And I want to subtract the previous value from the current value, by group ID date, to create this:</p>
  
  <pre><code>id      date        value   diff
  2380    10/30/12    21.01   0
  2380    10/31/12    22.04   1.03
  2380    11/1/12     22.65   0.61
  2380    11/2/12     23.11   0.46
  20100   10/30/12    35.21   0
  20100   10/31/12    37.07   1.86
  20100   11/1/12     38.17   1.1
  20100   11/2/12     38.97   0.8
  20103   10/30/12    57.98   0
  20103   10/31/12    60.83   2.85
  </code></pre>