Enable end-to-end tests

* Run via `grunt test:e2e`
* Runs by default on Chrome (after `npm install`)
* Test locally on more browsers by passing `PROTRACTOR_BROWSERS`, e.g.:

    PROTRACTOR_BROWSERS=chrome,firefox,safari grunt test:e2e

* Note, if you're running under `tmux`, prepend `reattach-to-user-namespace`:

    reattach-to-user-namespace grunt test:e2e

* Travis setup to run e2e tests via [Sauce Labs][1] (only on non-forks)

[1]: https://saucelabs.com/u/sense-ebola-dash