Accessibility primitive: get element(s) by accessibility label

[cookiecrook]

Accessibility primitive: get element(s) by accessibility label.

In the F2F today, Simon Pieters and Val Young presented a case for tying more WebDriver tests to Accessibility underpinnings. The strongest idea I heard was a need for an element getter that would allow an author to select an element by its accessible label. It's possible the label portion of #1439 will be a prerequisite.

[jugglinmike]

See gh-1440 for more on the motivation and implementation concerns