Added basic VoiceOver support to DTAttributedTextContentView.
Modified demo text view controller to hide non-visible content views to avoid conflicts with VoiceOver.
Invalidate accessibility elements when explicitly setting the layout frame.
Created view proxy for accessible attachments as subviews.
Merge upstream.
Re-added accessibility files to project.  Documented DTAccessibilityViewProxy
Documented DTAccessibilityElement.  Modified the designated initializer.
Documented layout frame accessibility element generator.
Added some comments.  Implemented hash/isEqual in view proxy.
Merge pull request #412 from x3dhydra/accessibility

Basic VoiceOver Support