Add Cross Browser Testing

[boldtrn]

There are tools that allow automated testing in different browsers and OS:
https://saucelabs.com/open-source
https://www.browserstack.com

Both are free for open-source software. Would be nice to know in which browsers the lib works and in which it doesn't. I think we need to write Selenium tests for this. Maybe in combination with the demo (I have no idea, haven't tried any of these tools).