Windows support in setup.py is dropped because it's not tested and probably does
not work anyway.

- start_new is a classmethod that creates Timeout and starts it
- start() will now raise AssertionError if called when already started()
- although it's OK to do "with t" even if t is already started
- when start_new is passed a Timeout instance it simply returns it;
  this allows to customize timeout exception in many places by passing Timeout
  instance instead of number of seconds