implement account cancellation

Implements #54. This takes the three scripts involved in account cancellation--
final-gift, untip, and deactivate--and moves them pretty much unchanged (except for removing the hashbang and print_function import) into methods on the Participant object. It also roughs in a UI.

![screen shot 2014-05-16 at 1 46 55 am](https://cloud.githubusercontent.com/assets/134455/2993701/8e05c888-dcbd-11e3-9254-a9b3e65099a0.png)

![screen shot 2014-05-16 at 1 47 36 am](https://cloud.githubusercontent.com/assets/134455/2993704/9b9b4b08-dcbd-11e3-8c26-a27b675e7c89.png)