Fix leak of os.pipe() file descriptors

Moved pipe creation into ConnectionHandler.start() and closes FDs
when zk_loop ends (triggered by stop()).
Fix test for python3
Add client close() method to release pipe FDs

And guard against hitting a closed pipe in _call()
Close clients in testing harness
Merge pull request #55 from python-zk/close_pipe

Fix leak of os.pipe() file descriptors