Make aui-datepicker accessible

• Type: Sub-Task
• Status: Closed
• Priority: Minor
• Resolution: Completed
• Affects Version/s: 2.0.x, master, master-deprecated, 3.0.x, 3.1.x
• Fix Version/s: master
• Component/s: DatePicker
• Labels: None
• Git Pull Request: https://github.com/jonmak08/alloy-ui/pull/370

Keyboard:
Currently cannot select dates on the calendar with keyboard

Expected keyboard behavior:

• Left: Move focus to the previous day. Will move to the last day of the previous month, if the current day is the first day of a month.
• Right: Move focus to the next day. Will move to the first day of the following month, if the current day is the last day of a month.
• Up: Move focus to the same day of the previous week. Will wrap to the appropriate day in the previous month.
• Down: Move focus to the same day of the following week. Will wrap to the appropriate day in the following month.
• Enter/Space: Select date

Aria:
Add aria-selected to the selected <td> on the datepicker calendar

Optional:
Add aria-live to announce the selected date