title: Pandas dataframe resample and count events per day
body: <p>I have a dataframe with time-index. I can resample the data to get (e.g) mean per-day, however I would like also to get the counts per day. Here is a sample:</p>
  
  <pre><code>import datetime
  import pandas as pd
  import numpy as np
  
  dates = pd.date_range(datetime.datetime(2012, 4, 5, 11, 
  0),datetime.datetime(2012, 4, 7, 7, 0),freq='5H')
  
  var1 = np.random.sample(dates.size) * 10.0
  var2 = np.random.sample(dates.size) * 10.0
  df = pd.DataFrame(data={'var1': var1, 'var2': var2}, index=dates)
  
  df1=df.resample('D').mean() 
  </code></pre>
  
  <p>I'd like to get also a 3rd column 'count' which counts per day:</p>
  
  <pre><code>count
  3
  5
  7
  </code></pre>
  
  <p>Thank you very much!           </p>