WIP: Integration tests with saucelabs

As mentioned in #940, I've attempted to setup integration testing in Travis CI with Saucelabs. I've managed to get the correct configuration and the first green build. 

Before I start writing some real tests, I'd like to hear your thoughts and comments about this setup.

Ideas:
* run integrations tests only in CI (or locally, but only if you have saucelab's username and key)
* grunt 'test' task executes a fresh build and spins up a local server before running the tests 
* use mocha test runner (+ great mocha-as-promised addon)
* chai should-style assertions
* use TodoMVC app for testing, both "classic" and with AMD-style

Todo:
- [ ] run tests in multiple browsers (which one?)
- [ ] include todo app with requirejs
- [ ] and of course, write some serious tests 

What I found interesting is that Soucelab can be used not only for running integration tests, but also for "standard" unit tests - in multiple browsers. In the future, we could consider moving all tests from travis to saucelab grid and run every test in multiple browsers.

*Edit:* not sure why travis didn't use the new config to build this PR. Here's a correct build from my fork: https://travis-ci.org/lukaszfiszer/backbone.marionette/builds/20499443