Add selenium tests via saucelabs

Before merging this please change the account data in 1f292656a1440dd96e7d408e4a717cc6aae75965.
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

(Please try in branch first, seems like travis might not use the full yml file from PRs)