title: pydot and graphviz error: Couldn't import dot_parser, loading of dot files will not be possible
body: <p>When I run a very simple code with pydot</p>
  
  <pre><code>import pydot
  graph = pydot.Dot(graph_type='graph')
  
  for i in range(3):
  
    edge = pydot.Edge("king", "lord%d" % i)
    graph.add_edge(edge)
  
  vassal_num = 0
  for i in range(3):
    for j in range(2):
      edge = pydot.Edge("lord%d" % i, "vassal%d" % vassal_num)
      graph.add_edge(edge)
      vassal_num += 1
  
  graph.write_png('example1_graph.png')
  </code></pre>
  
  <p>It prints me the error message:</p>
  
  <pre><code>Couldn't import dot_parser, loading of dot files will not be possible.
  </code></pre>
  
  <p>I'm using python 2.7.3</p>