Added ability to set date range for datetimepicker and minimum minute interval for time picker

Set the date range by calling: datePickerDialog.setDateRange(startYear, startMonth, startDay, endYear, endMonth, endDay);
Days outside this period will be disabled.

The minute interval restricts you to select every 1/5/10/15 etc minutes (defaults to 1).
The minute interval can be changed for the time picker by calling setMinutesInterval(val);