Daemon mode/absolute path

Added init script and changes to run the server as a daemon on system startup.  You need to specify an absolute path in config.js.  To use the init script modify it based on your system settings (see init script comments), copy it to /etc/init.d/grblweb, and run "update-rc.d grblweb defaults."
Go to zero button

Added a button that returns to 0,0,0 on working coordinates.  USE WITH CAUTION!  After a reset this will go somewhere else entirely.