Preliminary Saucelabs support

Cleanup required, but functional. No tests yet.
Try it out using:

```
browserify some/test.js | DEBUG=smokestack smokestack -b saucelabs
```

Note that you'll also need:
* process.env.SAUCE_USERNAME
* process.env.SAUCE_ACCESS_KEY

/cc @timoxley
tests for saucelabs
update README.md
proper API/CLI support for sauce
add test for clean closing

ensures that smokestack cleans up after itself and can
avoid using `process.exit`
add --help
add CLI flags for sauce labs username/key
Note on using Sauce Labs