Add keyboard navigation to single date picker

This only modifies keyboard input for the single date picker. It will have no effect on the range picker at all.

Problems solved:
1) Tabbing out of the datepicker didn't close it making keyboard navigation of the form impossible
2) There was no way to pick a date with the keyboard instead of moving the cursor to the desired location and editing the value in the field, which takes a long time

Adds: 
1) Tab to change focus now applies the datepicker and moves to the next field.
2) Up and Down arrow keys adjust date up and down a day, respectively
3) Use "Enter" to apply the date and close the datepicker.