title: dplyr Not Respecting the Group_By
body: <p>I have a simple example using <code>dplyr</code> (version 0.2)</p>
  
  <p>I want a cumulative sum of var1 by ID. It works fine with ddply from plyr but not the new package. Is this a user error? If so can anyone point me in the right direction?</p>
  
  <pre><code>  ID&lt;-c(1,1,1,1,2,2,3,4,4,4,4)
      var1&lt;-c(32,55,22,12,34,21,23,42,11,9,20)
      df&lt;-data.frame(ID=ID,var1=var1)
      df
  
  #does not create cumsum by ID    
  IDs&lt;-group_by(df,'ID')
  transform(IDs,cumsum=cumsum(var1))
  
     ID var1 cumsum
  1   1   32     32
  2   1   55     87
  3   1   22    109
  4   1   12    121
  5   2   34    155
  6   2   21    176
  7   3   23    199
  8   4   42    241
  9   4   11    252
  10  4    9    261
  11  4   20    281
  
  #works correctly
  ddply(.data=df, .variables=('ID'),.fun=transform,cumsum=cumsum(var1))
  
  
     ID var1 cumsum
  1   1   32     32
  2   1   55     87
  3   1   22    109
  4   1   12    121
  5   2   34     34
  6   2   21     55
  7   3   23     23
  8   4   42     42
  9   4   11     53
  10  4    9     62
  11  4   20     82
  </code></pre>