Don't use "instaclick/php-webdriver" in API Client implementation

[aik099]

Currently, the API Client classes are using the PHP classes (not WebDriver itself) of the instaclick/php-webdriver dependency, which will cause problems when we decide to switch to another default Mink session driver (not selenium2).

We need to create the abstract AbstractAPIClient class with cURL communication support and let actual clients (listed in examples) communicate through it.

Examples:

• the BrowserStackAPIClient is using ServiceFactory::getInstance()->getService('service.curl') to make cURL requests
• the SauceLabsAPIClient is using SauceRest class to communicate with SauceLabs