title: How to drop rows of Pandas DataFrame whose value in a certain column is NaN
body: <p>I have this <code>DataFrame</code> and want only the records whose <code>EPS</code> column is not <code>NaN</code>:</p>
  
  <pre><code>&gt;&gt;&gt; df
                   STK_ID  EPS  cash
  STK_ID RPT_Date                   
  601166 20111231  601166  NaN   NaN
  600036 20111231  600036  NaN    12
  600016 20111231  600016  4.3   NaN
  601009 20111231  601009  NaN   NaN
  601939 20111231  601939  2.5   NaN
  000001 20111231  000001  NaN   NaN
  </code></pre>
  
  <p>...i.e. something like <code>df.drop(....)</code> to get this resulting dataframe:</p>
  
  <pre><code>                  STK_ID  EPS  cash
  STK_ID RPT_Date                   
  600016 20111231  600016  4.3   NaN
  601939 20111231  601939  2.5   NaN
  </code></pre>
  
  <p>How do I do that?</p>