NullEndpoint for running unit test without Dispersy communicating.
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