Have our Sauce client set a session id property when instantiated

[nadvolod]

It's a little annoying to be doing something like this as a user:

        WebDriver driver = new RemoteWebDriver(new URL(SAUCE_REMOTE_URL), caps);
        SessionId sessionId = ((RemoteWebDriver) driver).getSessionId();

It would be way easier if our client could automatically set this property and then we could use it like this:

        SauceHelper sauceHelper = new SauceHelper(driver);
sauceHelper.getSessionId();