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 it without having to run as root.
update the docs for the new endpoint_owner functionality
add an entry to the contributors list for me