Supporting both python 2 and python 3 + fixed "threads can only be started once" issue

- _started is used by Thread class in python 3, so I renamed it to _running
(in python 2 Thread used __started, not _started)
- In constructor, right after calling Thread.start method, setting _running
to True, so the next call to Thread.show wouldn't raise "threads can only be started once"
error