Add selenium tests via saucelabs

Before merging this please change the account data in 707590227f8577a06231aeb42eab27a981cd4a12.
In order to encrypt the account data see [here](http://docs.travis-ci.com/user/encryption-keys/) (username has to be lower-case as I noticed).

Compare how it looks here:
Successful build: https://travis-ci.org/Sebmaster/jsdom/builds/36069070
Breaking build: https://travis-ci.org/Sebmaster/jsdom/builds/36069886