The calendar is missing accessibility properties

[barettjfed]

Describe the bug According to the ARIA Authoring Practices guide for W3C the calendar popup should have role="dialog" and aria-modal="true". Without these in place screen readers will not have trapped focus.

To Reproduce Steps to reproduce the behavior:

1. Go to any react-datepicker example
2. Notice that they are missing these properties

Expected behavior role="dialog" and aria-modal="true" are present on the calendar popup.