title: python pandas - group by date and count
body: <p>I have the below dataframe. Date in DD/MM/YY</p>
  
  <pre><code>Date                        id     
  1/5/2017 2:00 PM            100
  1/5/2017 3:00 PM            101
  2/5/2017 10:00 AM           102
  3/5/2017  09:00 AM          103
  3/5/2017  10:00 AM          104
  4/5/2017  09:00 AM          105 
  </code></pre>
  
  <p>Need output such a way that , able to group by date and also count number of Ids per day  , also ignore time. o/p new data frame should be as below </p>
  
  <pre><code>DATE           Count
  1/5/2017        2           -&gt; count 100,101
  2/5/2017        1
  3/5/2017        2
  4/5/2017        1
  </code></pre>
  
  <p>Need efficient way to achieve above.</p>