Reduce reliance on fields (e.g., _deviceId)

[erodewald]

I ran across an issue where the library would not work for my app which actually supports housing multiple users.

My app operates in a stateless manner, with configuration values passed between steps. Initiating pairing being one step - however 10 users could have initiated pairing through the same app instance before the original user has moved to the next step (entering their pairing code/pin). Unfortunately, the library leverages fields to pass those values around (_deviceId, _deviceName, _pairingRequestToken). I got around it by modifying a few functions.

@@ -114,12 +114,12 @@ let SMARTCAST = function smartcast(host, authKey) {
          * @param {string} pin The PIN displayed on the smartcast device
          * @return {Observable}
          */
-        pair: (pin) => {
+        pair: (pin, deviceId, pairingRequestToken) => {
             let data = {
-                DEVICE_ID: _deviceId,
+                DEVICE_ID: deviceId || _deviceId,
                 CHALLENGE_TYPE: 1,
                 RESPONSE_VALUE: pin,
-                PAIRING_REQ_TOKEN: _pairingRequestToken
+                PAIRING_REQ_TOKEN: pairingRequestToken || _pairingRequestToken
             };
             return sendRequest('put', host + '/pairing/pair', null, data).then((data) => {
                 if (data && data.STATUS.RESULT === 'SUCCESS') {

They remain optional, in the event that (most people) aren't going to need this functionality. It's kind of a band-aid, let me know what you think.