[#15] Added approving for expenses (allow user to validate which expenses are already done).

* when user adds expense with future date, it will be marked as undone expense.
* user is able to approve which expenses are already done (by clicking on button). After click on button, date will be set to current day.
* expenses with date in past, but without validation of the user, will still wait for approval.

![Przechwytywanie](https://f.cloud.github.com/assets/619324/143623/36f8a452-73b9-11e2-98dd-2a929aaa3684.PNG)
![Przechwytywanie2](https://f.cloud.github.com/assets/619324/143624/3714f684-73b9-11e2-834c-5d6d8eb50682.PNG)