Datepicker: Use Globalize 1.0.0

- Move external/date.js -> ui/calendar/date.js and UMDify it;
- Create a locale option as discussed during Chicago's meeting:
  - [String] A String with locale value, e.g. "en"; or
  - [Object] An object with all the formatters.
  TODO: Use the Globalize to-be-created generators instead of these wrap
  functions.