Support setting timezone

[daniel-sc]

As acknowledged by support staff and reflected in the readme (see commit 624494e) it is currently not possible to set the timezone of the browser. This makes any tests that deal with dates unstable. For a good reason this is supported for Selenium tests - so it should be supported for Karma tests as well!

I don't know if the launcher can implement this, or this needs to be supported bei Browserstack somehow, but this situation is very annoying, so I'd like to track this publicly here.

See as well here https://stackoverflow.com/questions/43630813/is-it-possible-to-set-the-timezone-in-browserstack and #66 (comment)