Move dependency on SauceTunnel to a separate module

I've extracted the SauceTunnel code to its own module, and removed its dependency on Grunt (instead emitting log events). The immediate need for the extraction was so that I could use the code in [grunt-mocha-webdriver](https://github.com/jmreidy/grunt-mocha-webdriver), but in general I think it makes sense for the SauceTunnel code to exist independently.