Mws.open implementation

In the QTC Developer Documentation site is the following Node.js snippet:

```javascript
/** 
 * client.js
 * This script will open socket connection and listen for incoming messages
 */
var qtc = require("qtc");
var mws = new qtc.Mws("YOUR_WEBSOCKET_ADDRESS", "YOUR_WEBSOCKET_ID");

// add event listener for incoming messages
mws.on("connect", function(){
    console.log("Connected to MWS WebSocket!");
    console.log("Listening for messages...");
})

mws.on("message", function(data){
    console.log(data);
})

// open socket connection 
mws.open();
```
https://developer.qtcloudservices.com/mws/getting-started

However Mws.open method is missing in current the implementation. This PR implements Mws.open() method like it is documented.