Integrate "pa11y-ci" and other accessibility-related testing

[nfreear]

..

• https://github.com/pa11y/pa11y-ci
• http://cruft.io/posts/automated-accessibility-testing-node-travis-ci-pa11y/
• https://marcysutton.github.io/a11y-and-ci/#/
• https://github.com/AccessLint/accesslint-ci
• https://github.com/apps/accesslint
• Search: https://google.co.uk/search?q=CI+accessibility+testing#

---

• Loads of "errors" in build !! ~ https://travis-ci.org/nfreear/gaad-widget/builds/340965313

---

[iet:10304380]