title: Sum rows of data frame for each unique pair of values
body: <p>I have got a data frame that looks like this:</p>
  
  <pre><code> &gt; df
             date day        x    y   z
  [1]  2014-02-24 Thursday   100  83  39
  [2]  2014-02-26 Wednessday 486 189 171
  [3]  2014-02-26 Wednessday 426 427 428
  [4]  2014-02-26 Wednessday 423 286 195
  [5]  2014-02-27 Thursday   374 339 229
  [6]  2014-02-27 Thursday   744 496 489
  </code></pre>
  
  <p>I want to sum <code>x</code>, <code>y</code>, and <code>z</code> for each unique pair of <code>date</code> and <code>day</code> variables. It should look like this:</p>
  
  <pre><code>           date day        x    y   z
  [1]  2014-02-24 Thursday   100  83  39
  [2]  2014-02-26 Wednessday 1335 902 794
  [6]  2014-02-27 Thursday   1118 835 718
  </code></pre>
  
  <p>Any help please?? thanks a lot!!</p>