update httpd plist to mimic Apple system Apache

This is in response to https://github.com/jimbojsb/launchrocket/issues/30. The core of the issue was that apachectl is not a launchd-compatible way of starting and stopping httpd, as launchd must control the daemonization. I copied the startup commands from Apple's built-in httpd plist, and launchctl can now stop and start httpd properly.