Fixes and a live overlay test.

Fix to callback: https://github.com/boudewijn-tribler/dispersy/commit/5282b1483ec44ae6e5d22708192243326f187c13

Fix to Dispersy yield behaviour: https://github.com/boudewijn-tribler/dispersy/commit/eddf9f0cf2723e17f3e09772872b3ae601b4be10

Fix for tracker bug: https://github.com/boudewijn-tribler/dispersy/commit/4325692fe7865518e8a4690c2ea6efccd10d77a3