NullEndpoint for running unit test without Dispersy communicating.
Merge pull request #57 from boudewijn-tribler/endpoint

NullEndpoint for running unit test without Dispersy communicating.
Merge pull request #60 from boudewijn-tribler/code-cleanup

Callback.call will now propagate the exception with the callstack.
Merge remote-tracking branch 'remotes/upstream/devel' into code-cleanup
Fixes issue when no candidates are available to introduce.