Make Opera driver support also the new Blink based Opera

Support for Chromium based Opera. Presto support is retained. The selenium.webdriver.opera.WebDriver handles both PRESTO and CHROMIUM engines for desktop platforms, the default is PRESTO. Android is supported only by the new OperaDriver.

@andreastt suggested that it might be good to introduce separate driver class in selenium.webdriver. __init__.py. If not, it might be good to add desktop browser detection (Presto or Chromium based) - it can be done using 'opera --version' commands and other information.

Why was this split out into a method? This seems superfluous...