uninstall_mp_handler() doesn't handle >1 handlers

[knightofni-i]

trafficstars

Need to use for handler in list(logger.handlers): instead of current for handler in logger.handlers: When I get some time, I will fork project and make a pull request.

[knightofni-i]

Additionally, we should call self.manager.shutdown() in MultiProcessingHandler.close()

[jruere]

This was contributed and has no tests. I'll work on it eventually. Alternatively, if you want, you can submit a PR with tests and the fix.

[jruere]

This was fixed without tests. Will be released on the next version.