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

Allows to use any Sahi server for testing SahiDriver itself, not only `localhost`, as was before.

Also setting for server path mapping (see https://github.com/Behat/Mink/pull/342 pull request).

Similar functionality for Selenium2Driver: https://github.com/Behat/MinkSelenium2Driver/pull/56