Added `MD_` prefix to all NSDate category methods
Accessibility & VoiceOver:
- Days now speak their weekday, month and year
- Weekday view is one accessibility element
- Invalid days will be suffixed with "dimmed"

- Fleshed out some more date convenience methods
Removed MD_ prefix
Merge pull request #11 from noahemmet/accessibility

Accessibility improvements & MD_prefix [IN PROGRESS]