title: Cannot create a directed graph using from_pandas_dataframe from networkx
body: <p>I'm learning networkx library and use twitter retweet directed graph data. 
  I first read the datasets into pandas df (columns are 'from','to','weight') and wanted to put a first 300 rows(retweet) into a graph using below code:</p>
  
  <pre><code>tw_small = nx.from_pandas_dataframe(edges_df[:300],source='from',
                                     target='to',edge_attr=True)
  </code></pre>
  
  <p>I thought that it correctly created a graph but when I run <code>tw_small.is_directed()</code>, it says <code>False</code>(undirected graph) and I drew a graph using <code>nx.draw()</code> but it doesn't show the direction either. </p>
  
  <p>Could someone help me find a correct way to make a directed graph? </p>
  
  <p>Thanks.</p>