title: Pandas Aggregate/Group by based on most recent date
body: <p>I have a DataFrame as follows, where Id is a string and Date is a datetime:</p>
  
  <pre><code>Id    Date
  1     3-1-2012
  1     4-8-2013
  2     1-17-2013
  2     5-4-2013
  2     10-30-2012
  3     1-3-2013
  </code></pre>
  
  <p>I'd like to consolidate the table to just show one row for each Id which has the most recent date.<br>
  Any thoughts on how to do this?</p>