Remove reference to last task and his arguments in engine to free memory.
Avoid circular references in StackContext and ExceptionStackContext.
Fix typo.
Fix options with multiple=True on python 3; add tests.
Add a --debug_gc flag to runtests to easily turn on extra logging.
Use add_callback to trigger ioloop in test.
Merge remote-tracking branch 'homm/master'
Use weakrefs instead of __del__ in reference cycle test.

This avoids an awkward and otherwise meaningless use of Task(None).