This statement wasn't true anymore
request cache doesn't use the callback anymore
community.py doesn't use the callback anymore
dispersytestclass.py doesn't use the callback anymore
node.py doesn't use the callback anymore
Update doctrings
Add a clear() method to the request cache so we can leave a completely
clean reactor when we stop a dispersy instance.
fix/simplify Community._download_master_member_identity() to not leave
pending tasks behind.
Test that the reactor is clean before and after each individual test execution.
Fully migrate dispersy.py away from the callback.
Migrate main.py to twisted
Port tracker.py to Twisted
Remove callback.py and its tests, not used anymore.
Revert/adapt "fix/simplify Community._download_master_member..." to a
static interval LoopingCall.

This reverts commit 0aaad467a7ef2f7cf2e4de0bef040462a9819ac2.
_pending_tasks is a dict now, implemented cancel_pending_task(),
_batch_cache doesn't store the Cache DelayedCall anymore as it was
stored in _pending_tasks too.
Typo and cleanup
Use a dict for the request timeout delayed calls
Fix the call on reactor thread decorator
Move the retrial loop to receive_messages so all the tests can benefit
from it.
Fix and move the reactor call decorator to util.py and refactor all the
unit tests to use it.