Add support for sauce connect as alternative to local selenium server

Let me know your thoughts on this - I wanted to have the ability to switch back and forth between running WD/mocha tests locally and in sauce labs (e.g. for development vs. continuous integration), so I modified the mocha-selenium task to support either or.

I've held off on making the relevant changes to the README file or writing any tests because I wanted to get your thoughts on this approach first - does this seem reasonable to you? I considered putting this functionality in selenium-launcher itself, but this made more sense to me.