title: Combine a dict of varied-length data frames into a single data frame, and pad with zeros to enforce dimensionality
body: <p>I have a collection of time Series in a <code>dict</code>:</p>
  <pre><code>my_dict = {
       ts_1 : df_1,
       ts_2 : df_2,
       ts_3 : df_3,
       ts_4 : df_4,
       ts_5 : df_5
  }
  </code></pre>
  <p>...where each <code>df</code> is of the form:</p>
  <pre><code>       yr_mo    number
  0   2014-12-01  2
  1   2015-01-01  42
  2   2015-02-01  7
  3   2015-03-01  15
  4   2015-04-01  18
  </code></pre>
  <p>I need to convert this <code>dict</code> of data frames into a single <code>df</code> of the form:</p>
  <pre><code>     2014-12-01    2015-01-01    2015-02-01    2015-03-01    2015-04-01
  0        2             42            7            15             18
  1.       7             44           33            38              9
  2.       3              6           43             9              3
  3.       8              3           77             3              9
  4.       1              7           79             2             10
  </code></pre>
  <p>Since these time Series are of different length, I would like to pad the shorter time Series with zeros, so that the dimensionality is the same for each Series.</p>
  <p>I have tried by doing the following:</p>
  <pre><code>df_collection = pd.DataFrame.from_dict(my_dict, orient='index')
  </code></pre>
  <p>...but this gives the incorrect format.</p>