Set 'annotation-id' data attr on highlight elements

This allows the implementation of hover styles for specific annotations
across multiple highlight elements, as requested in #232.

Fixes #232.

Backported from master to v1.2.x.

Conflicts:
	src/annotator.coffee
	test/spec/annotator_spec.coffee
Add tests and clean up document alternate ignores

This also backports #355.

Conflicts:
	src/plugin/document.coffee