title: Python Pandas: drop rows of a timeserie based on time range
body: <p>I have the following timeserie:</p>
  
  <pre><code>start = pd.to_datetime('2016-1-1')
  end = pd.to_datetime('2016-1-15')
  rng = pd.date_range(start, end, freq='2h')
  df = pd.DataFrame({'timestamp': rng, 'values': np.random.randint(0,100,len(rng))})  
  df = df.set_index(['timestamp'])
  </code></pre>
  
  <p>I would like to drop the rows that are between those 2 timestamps:</p>
  
  <pre><code>start_remove = pd.to_datetime('2016-1-4')
  end_remove = pd.to_datetime('2016-1-8')
  </code></pre>
  
  <p>How can I do that?</p>