Added support for setting and changing reminder due date and time

[andylin2004]

A previous issue discussed about adding the intelligent date and time setter like in the reminders app (see #17). This is a partial solution to that issue where users can change and set reminder due date and time manually (either below the textfield or via a popover that will apply changes once the user clicks out of the popover). English localization has also been applied for this.

Happy holidays! :)