added _instance to SetValue to support multi-instance devices

Hi Jonathan,
This patch is to let node-openzwave properly control multi-instance devices such as the Fibaro FGS-221 (double in-wall relay switch). It adds the missing "_instance" part of the OZW ValueId.

zwave.setValue(8, 37, 0, 1, true/false); // control 1st relay of node #8
zwave.setValue(8, 37, 0, 2, true/false); // control 2nd relay of node #8

This patch also tightens the search for ValueIds for switchOn/Off and setLevel so that they only apply affect the valueID with instance and index values of 0. This was necessary as a switchOn command affected both relays.