Suport of http-proxy and new method for getting screenshot as file

1. Method "Set proxy" could take a "Proxy" object created by browsermob-proxy or by any other way. Not fully tested yet, but checked for Firefox and Chrome. Troubles found with phantomJS (I suppose it connected with browsermob).
2. Method "getScreenShotAsFile()" allows simplify passing images to third-party tools (to allure reporting framework for instance).