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?
node is already on the system because we are using it for `start_test.js`. What I am saying here is that `package.json` could be a common file to find in repositories in general, even if it is a Meteor package repository not really using otherwise `package.json`. (Meteor packages use `package.js`.)