title: Combine multiple dataframes into one that sums their values according to the index
body: <p>I have several pandas dataframes, each with one column of ints in them, and I would like to create a new dataframe with the sum of their values at each index. Their indexes will have some overlapping entries and these are the indecies whose values I want to add together. If an index is found in only one dataframe I want the new dataframe (or series) to include that index and just use that one value as its value. This seems straight-forward but I can't figure it out and the documentation seems to focus on joining dataframes more so than combining their values.
  Basically, given two dataframes that look like this:</p>
  
  <pre><code>&gt;&gt;&gt; df1
     0
  a  3
  b  7
  d  2
  &gt;&gt;&gt; df2
      0
  c  11
  d  19
  </code></pre>
  
  <p>And I'd like to have the final output look like this:</p>
  
  <pre><code>&gt;&gt;&gt; df3
      0
  a   3
  b   7
  c  11
  d  21
  </code></pre>
  
  <p>Thanks in advance.</p>