Fix missing z-index on calendar popup with pickDate=false or pickTime=false options.

Hi,

I saw that the z-index style is missing from the dropdown-menu if either pickDate or pickTime option is set to false. I tried without success to built a jsFiddle to reproduce the problem.

For the record, I'm seeing this problem in a AngularJS app when opening a modal (from ui-bootstrap) that contains a datetimepicker.

Thank you for your work, it is much appreciated !