title: Adding two pandas dataframes
body: <p>I have two <code>dataframes</code>, both indexed by <code>timeseries</code>.  I need to add the elements together to form a new <code>dataframe</code>, but only if the index and column are the same.  If the item does not exist in one of the <code>dataframe</code>s then it should be treated as a zero.</p>
  
  <p>I've tried using <code>.add</code> but this sums regardless of index and column.  Also tried a simple <code>combined_data = dataframe1 + dataframe2</code> but this give a <code>NaN</code> if both dataframes don't have the element.</p>
  
  <p>Any suggestions?</p>