Continuous integration of browser tests

**Disclaimer:** this probably won't work right now as I'll possibly have to tweak the travis/saucelabs configuration a bit using this very pull request.

THis is a terrible mess right now, travis secure env vars are not working at all. Sorry for the noise.

**Update:**

Currently [blocked](https://trello.com/c/JqEiXlPJ) because [travis-ci doesn't allow to set and use secure env vars in pull requests](http://about.travis-ci.org/docs/user/build-configuration/#Secure-environment-variables):

> Please note that secure env variables are not available for pull requests. This is done due to the security risk of exposing such information in submitted code. Everyone can submit a pull request and if an unencrypted variable is available there, it could be easily displayed.

Options are:

- to move away from travis
- to investigate travis pro?
- to move away from sauce labs
- to expose our sauce labs creds publicly (hazardous) (no go)