Give a UIAccessibilityTraitSelected to the selectedButton

Currently, the selected date is the calendar view doesn't have the UIAccessibilityTraitSelected trait. It affects VoiceOver accessibility and testing using accessibility traits.

This change ensure the selected date always has the UIAccessibilityTraitSelected trait when displayed.
Merge pull request #13 from kemenaran/uiaccessibilitytraitselected

Give a UIAccessibilityTraitSelected to the selectedButton