title: Sum all values in a dataframe
body: <p>I am trying to sum all the values in a dataframe into one number.</p>
  
  <p>So for example with the dataframe</p>
  
  <pre><code>            BBG.XAMS.FUR.S_pnl_pos_cost  BBG.XAMS.MT.S_pnl_pos_cost
  date                                                               
  2015-03-23                    -0.674996                   -0.674997
  2015-03-24                    82.704951                   11.868748
  2015-03-25                   -11.027327                   84.160210
  2015-03-26                   228.426675                 -131.901556
  2015-03-27                   -99.744986                  214.579858
  </code></pre>
  
  <p>I would like the value 377.71658 returned. </p>
  
  <p>I have tried df.sum() but that only sums by column.</p>