Saucelabs support

This pull request is only so that we can do an internal code review. We will not get merge it, but send a pull request upstream.

@arunoda, we are adding Sause Labs support to travis-ci-meteor-packages. Would that be something you would be interested in merging in?
how it can be overridden for meteor apps? anyway. That's the only way for a meteor app.
But I think we can set it to generic app and use a makefile as well.
If so, we need to download node manually or use the node comes with meteor.