title: group list of objects by date and sort them time using rxjava
body: <p>I have a list of restaurants' reservations.
  I want to group them by day of the year and sort them by time on that day.
  How can I do so using rxjava?</p>
  
  <pre><code>List reservations;
  
  class Reservation {
     public String guestName;
     public long time; //time in milliseconds
  }</code></pre>
  
  <p>OUTPUT</p>
  
  <ul>
  <li>day 5th of March 2015
  <ul>
  <li>reservation for 20:00</li>
  <li>reservation for 22:00</li>
  </ul></li>
  <li>day 8th of of March 2015
  <ul>
  <li>reservation for 16:00</li>
  </ul></li>
  </ul>