title: Generating new table based on unique column values
body: <p>I have a <code>data.table</code> with the following form:</p>
  
  <pre><code>          date           action cnt
   1: 2016-06-14      PICK_APPLE   2
   2: 2016-06-15      PICK_APPLE  12
   3: 2016-06-16      PICK_APPLE   7
   4: 2016-06-17      PICK_APPLE   5
   5: 2016-08-10      PICK_APPLE   1
   6: 2016-08-11      PICK_APPLE   1
   7: 2016-08-14      PICK_APPLE   1
   8: 2016-08-17      PICK_APPLE   7
   9: 2016-08-19      PICK_APPLE   1
  10: 2016-08-21      PICK_APPLE   2
  11: 2016-08-22      PICK_APPLE   3
  12: 2016-08-23      PICK_APPLE  12
  13: 2016-08-24      PICK_APPLE   6
  14: 2016-08-30      PICK_APPLE   4
  15: 2016-08-31      PICK_APPLE   4
  16: 2016-09-05      PICK_APPLE   8
  17: 2016-09-08      PICK_APPLE   1
  18: 2016-08-23     PICK_ORANGE   1
  19: 2016-08-31     PICK_ORANGE   2
  20: 2016-09-05     PICK_ORANGE   2
  21: 2016-08-05       ATE_APPLE   1
  22: 2016-08-14       ATE_APPLE   1
  23: 2016-08-17       ATE_APPLE   1
  24: 2016-08-18       ATE_APPLE   1
  25: 2016-08-30       ATE_APPLE   3
        date           action     cnt
  </code></pre>
  
  <p>And I want to create a table with the column names as the unique values from the <code>action</code>column and the row names as the unique values within the <code>date</code> column. E.g the result should be something like:</p>
  
  <pre><code>          date           PICK_APPLE PICK_ORANGE ATE_APPLE
   1: 2016-06-14               2
   2: 2016-06-15              12
   3: 2016-06-16               7
   4: 2016-06-17               5
  21: 2016-08-05                                      1
   5: 2016-08-10               1
   6: 2016-08-11               1
   7: 2016-08-14               1                      1
   8: 2016-08-17               7                      1
  24: 2016-08-18                                      1
   9: 2016-08-19               1
  10: 2016-08-21               2
  11: 2016-08-22               3
  12: 2016-08-23              12         1
  13: 2016-08-24               6
  14: 2016-08-30               4
  15: 2016-08-31               4         2
  16: 2016-09-05               8
  17: 2016-09-08               1
  20: 2016-09-05                         2
            date           PICK_APPLE PICK_ORANGE ATE_APPLE
  </code></pre>
  
  <p>Is there a standard way of doing this, or will I need to do this by looping through each of the <code>(date,action)</code> combinations?</p>