Bok-choy

* Added BrowserQuery is_focused method and focused property
* Ability to include custom rules in an axe-core accessibility audit

* Allow A11yAudits to run on browsers other than phantomjs

* Fix for generating a11y API documentation.

* Added check the loading state of the document in wait_for_page to reduce flakiness due to synchronization/timing in tests.
* Refactored accesibility auditing feature. Two accessibility auditing rulesets now available: axe-core ruleset (default) and google's accessibility dev tools ruleset.
* Update browsermob proxy version to the latest

* Bugfix in wait_for_ajax to check that jQuery is defined

* Implement distinction between an element being invisible vs not-present