Improve Accessibility

[MosheBerman]

I haven't done a full accessibility audit yet, but some areas that definitely need work:

• The next/back buttons in the header view are currently custom drawn arrow images with a touch gesture recognizer. Wrapping the images in UIButtons and setting some accessibility attributes will probably be a huge step in the right direction.
• Preferred Content Size adoption in the header view and the cells would be awesome. (See Improve Landscape Mode #121 for a discussion about cell sizes. It's relevant to custom cell content that doesn't necessarily fit cell sizes that are fixed.)
• The calendar view's intrinsicContentSize is also probably going to be relevant here.
• An audit for VoiceOver support.
• An audit on scrubbing.

As an addition to all of the awesome localization and international enhancements made in 5.0.0, this would be really great.