title: Summing values in dictionary of dictionaries
body: <p>I got something like this:</p>
  
  <pre><code>&gt; d1 = {'System tests': {'failed': 5, 'passed': 0, 'total': 5},
         'Func tests': {'failed': 5, 'passed': 0, 'total': 5}}
  
  &gt; d2 = {'System tests': {'failed': 1, 'passed': 1, 'total': 2}, 
          'Func tests': {'failed': 3, 'passed': 2, 'total': 5}}
  
  &gt; d3 = {'System tests': {'failed': 0, 'passed': 0, 'total': 0}, 
          'Func tests': {'failed': 1, 'passed': 0, 'total': 1}}
  </code></pre>
  
  <p>I would like to sum values 'failed', passed and total into one dictionary</p>
  
  <p>so the output should be like this:</p>
  
  <pre><code>d4 = {'System tests': {'failed': 6, 'passed': 1, 'total': 7}, 
         'Func tests': {'failed': 9, 'passed': 2, 'total': 11}
  </code></pre>
  
  <p>What is the easiest solution to do such thing?</p>
  
  <p><strong>I can use basic libraries, except collections.</strong></p>
  
  <p>The solution must be generic, for example if some other dictionaries would appear in the future</p>