org-clock-sum saves details of the clocked time for org-invoice-report

Add such an invoice section to your org file:

and call org-invoice-report on it. The result is

| Task / Date |     Time |    Price | Activity                                 |
|-------------+----------+----------+------------------------------------------|
|  16.01.2014 | 10:10:00 | 12:50:00 | Doing something useful 1                 |
|  17.01.2014 | 08:20:00 | 12:00:00 | Another useful activity                  |
|  20.01.2014 | 08:15:00 | 12:10:00 | Procrastinating while looking busy       |
|  21.01.2014 | 10:30:00 | 11:45:00 | Trying out something new                 |
|  22.01.2014 | 10:30:00 | 12:30:00 | Working hard without achieving anything  |
|-------------+----------+----------+------------------------------------------|
|      Total: | 1d 16:30 |     0.00 |                                          |

Change-Id: Idb8f55aa3ec8c0c0d8257ea340150c2f82bcfdce