title: Activity log into Pandas Series
body: <p>I have as input a csv with (USERID, Datetime)
  In order to analyze the activity of my users I would like to generate a Pandas Series with the index being a date, and columns being my users (multiple series).
  The values would be the sum of the activities of each user aggregated over that date (for instance: Day).</p>
  
  <p>Thanks in advance.</p>