Allow to specify alternative (not always 'localhost') Selenium server url

Allows to use any Selenium server for testing `SeleniumDriver` itself, not only `127.0.0.1`, as was before.

Also setting for server path mapping (see https://github.com/Behat/Mink/issues/342 pull request).

Similar PR was made to Selenium2Driver: https://github.com/Behat/MinkSelenium2Driver/pull/56