Implement beforeShowDay callback during render

This is similar to the beforeShowDay callback found in the DatePicker
widget from jquery UI
(http://api.jqueryui.com/datepicker/#option-beforeShowDay). The only
omission is a custom tooltip for the date.

The beforeShowDay callback can be supplied when creating an instance of
the DateTimePicker.
Examples for beforeShowDay callback in index.html
Note for datetimepicker beforeShowDay callback
Remove console output from index.html
Remove call to beforeShowDay if not user-defined

Rather than calling the function regardless if the user wished to
perform a check on each date, only call the method if the user
explicitly defined it in the options object passed to datetimepicker.
Fix list isshue
Fix list issue