title: Given a flat list of (parent,child), create a hierarchical dictionary tree
body: <p>I have a list with tuples that containing names. These names are parent and child names and I want to create a hierarchical dictionary tree with their names.
  For example I have the following list:</p>
  
  <pre><code>[('john','marry'),('mike','john'),('mike','hellen'),('john','elisa')]
  </code></pre>
  
  <p>and I want to create this:</p>
  
  <pre><code>{
      'mike':{
          'john':{
              'marry':{}
              'elisa':{}
           }
           'hellen':{}
          }
  }
  </code></pre>