new feature for setting the ipc endpoint socket owner

- Added a new config option called 'endpoint_owner' that must be set to
a unix user name
- When the endpoint is an ipc connection and endpoint_owner is set, then
the ipc socket file will be chown'd to the endpoint_owner user value.
This allows circus to run as root but allow certain processes to send
commands to add processes without having to run as root.

Conflicts:
	circus/arbiter.py