Method for getting WebDriver’s Session ID

Nowadays Selenium became so popular, that even hosted solutions with premade Selenium servers exists. Here are few examples:
* https://saucelabs.com/
* http://www.browserstack.com/

To interact with them through their API an ID of session, started using WebDriver protocol, is needed. For example we can update test status using it.

The code is pretty trivial: `$this->wdSession ? basename($this->wdSession->getUrl()) : '';` but to get the same from outside of a driver more code need to be written.

In this PR I've decided to simplify developers life, by adding `getSessionId` method to the driver.

@stof, I hope this PR can make it's way into upcoming release you're planning to make this weekend.