NullEndpoint for running unit test without Dispersy communicating.
Fixes NoneType error.

Sometimes result is None, i.e. not a Candidate instance.
Merge pull request #58 from boudewijn-tribler/bugfixes

Fixes NoneType error.
Merge pull request #57 from boudewijn-tribler/endpoint

NullEndpoint for running unit test without Dispersy communicating.
Callback.call will now propagate the exception with the callstack.
Merge pull request #60 from boudewijn-tribler/code-cleanup

Callback.call will now propagate the exception with the callstack.
Modified yield iterators, now all of them end + walk and introduction
iterators are now simply returning the first one.
Merge pull request #63 from NielsZeilemaker/new_iterators

Modified yield iterators, now all of them end + walk and introduction
Removed obsolete code.
Iterate incoming intro-req twice.

This will ensure that even the first message in the batch will be
introduced to a candidate.
Unittests for candidates and walker.
Extended the candidate unittests
Commentary changes.
Forgot to add import
Merge pull request #64 from boudewijn-tribler/unittest

More unit tests and small fix
Merge branch 'devel' of ssh://github.com/Tribler/dispersy into
probability_unitttests

Conflicts:
	tests/test_candidates.py
More candidate/walker related counters.
Output debug statistics when logger is enabled.
Merge remote-tracking branch 'remotes/upstream/devel' into bugfixes
Spellcheck.