added close method so it will play nice with Node-RED

Hi,

I needed to add a close call so Node-RED can handle flow updates and shutdowns cleanly. If you can accept this and update the npmjs.org package it will save me having to point to the fork in the Node-RED Powermate node.

Thanks.
It would be nice to fail, if the HID is closed ... is there a reason why the error is swallowed here?