feat(sensors): add battery level

Also refactored some common parts for sensors. Refactored to parse data in payload and set it as object

utils.js

@@ -0,0 +1,33 @@
+
+
+module.exports = {
+    computeBatteryLevel: function(voltage) {
+        /*
+          When full, CR2032 batteries are between 3 and 3.4V
+          http://farnell.com/datasheets/1496885.pdf
+        */
+        return Math.min(Math.round((voltage - 2200) / 14), 100);
+    },
+    setStatus: function(node, data) {
+        if (data.voltage) {
+            var batteryPercent = Math.min(Math.round((data.voltage - 2200) / 14), 100);
+            var status = {
+                fill: "green", shape: "dot",
+                text: "battery - " + batteryPercent + "%"
+            };
+
+            if (data.voltage < 2500) {
+                status.color = "red";
+            } else if (data.voltage < 2900) {
+                status.color = "yellow";
+            }
+            node.status(status);
+        }
+    },
+    prepareFullDataOutput: function(payload) {
+        if(payload.data.voltage) {
+            payload.data.batteryLevel = this.computeBatteryLevel(payload.data.voltage);
+        }
+        return payload;
+    }
+}