Closes #102

Using this script:

1. Copy scripts/launchd/io.gogs.web.plist into /Library/LaunchDaemons

2. The script assumes Gogs is running under 'gogs' user and group, modify
   /Library/LaunchDaemons/io.gogs.web.plist if you want to user different user.

3. The script assumes Gogs is installed in /Users/git/gogs, modify
   /Library/LaunchDaemons/io.gogs.web.plist if you installed Gogs in different
   location.

4. Once you are sure that running Gogs manually via `gogs web` works fine, run
   it as a launchd service with:

       sudo launchctl load -F /Library/LaunchDaemons/io.gogs.web.plist

From now on launchd will ensure Gogs is running, eg. when system is restarted.