Configure functional tests (for local and SauceLabs -browsers) using testem, mocha and chai

@tuukka knows what this is about.

Patch for `index.html` could be replaced by copying `index.html` into `tests`-directory and adding the required JavaScript-dependencies and test initialization code at the bottom of `body`-tag. The current approach, however, avoids the extra work of keeping that in sync with the main `index.html`.