fix(network): fix stun error on startup if no network available

Fixed #7

src/background/checkInternet.js

@@ -0,0 +1,10 @@
+// https://stackoverflow.com/questions/15270902/check-for-internet-connectivity-in-nodejs
+module.exports = function checkInternet(cb) {
+    require('dns').lookup('google.com',function(err) {
+        if (err && err.code == "ENOTFOUND") {
+            cb(false);
+        } else {
+            cb(true);
+        }
+    })
+}