title: Pandas reindex dates in Groupby
body: <p>I have a dataframe with sporadic dates as the index, and columns = 'id' and 'num'.  I would like to <code>pd.groupby</code> the 'id' column, and apply the reindex to each group in the dataframe.</p>
  
  <p>My sample dataset looks like this:</p>
  
  <pre><code>            id  num
  2015-08-01  1   3
  2015-08-05  1   5
  2015-08-06  1   4
  2015-07-31  2   1
  2015-08-03  2   2
  2015-08-06  2   3
  </code></pre>
  
  <p>My expected output once <code>pd.reindex</code> with <code>ffill</code> is:</p>
  
  <pre><code>            id  num
  2015-08-01  1   3
  2015-08-02  1   3
  2015-08-03  1   3
  2015-08-04  1   3
  2015-08-05  1   5
  2015-08-06  1   4
  2015-07-31  2   1
  2015-08-01  2   1
  2015-08-02  2   1
  2015-08-03  2   2
  2015-08-04  2   2
  2015-08-05  2   2
  2015-08-06  2   3
  </code></pre>
  
  <p>I have tried this, among other things to no avail:
  <code>newdf=df.groupby('id').reindex(method='ffill')</code> 
  Which returns error:<code>AttributeError: Cannot access callable attribute 'reindex' of 'DataFrameGroupBy' objects, try using the 'apply' method</code></p>
  
  <p>Any help would be much appreciated</p>