title: counting total occurrence of each session id and start and end time per SessionID
body: <p>suppose i have a dataframe</p>
  
  <pre><code>&gt; str(data)
  'data.frame':   2538 obs. of  5 variables:
   $ X        : int  1 2 3 4 5 6 7 8 9 10 ...
   $ SessionID: int  13307 21076 27813 8398 23118 12256 28799 11457 7542 19261 ...
   $ Timestamp: POSIXct, format: "2014-04-06 18:42:05" "2014-04-03 15:27:48" "2014-04-04 09:10:14" "2014-04-03 23:39:20" ...
   $ ItemID   : int  214684513 214718203 214716928 214826900 214838180 214717318 214821307 214537967 214835775 214706432 ...
   $ Price    : int  0 0 0 0 0 0 0 0 0 0 ...
  </code></pre>
  
  <p>and i want to count total occurrance of each SessionID and each session start and end time mean i want output like this</p>
  
  <pre><code>&gt; data
      session id     timestamp           price       
        1         2014-04-0618:42:05.822     0     
        1         2014-04-0618:42:06.800     1
        1         2014-04-0618:42:06.820     0
        2         2014-04-0315:27:48.118     0
        2         2014-04-0315:27:49.440     0
  
  &gt;  result   
  session id   session start and end time                        num of occurrence   
   1           2014-04-0618:42:05.822, 2014-04-0618:42:06.820       3
   2           2014-04-0315:27:48.118, 2014-04-0315:27:49.440       2
  </code></pre>