Datepicker and Calendar: WIP

This PR introduces the Calendar widget which is utilized in Datepicker widget. It also includes several improvements and changes as outlined in the wiki: http://wiki.jqueryui.com/w/page/12137778/Datepicker

Included changes and bugfixes: 
* Introduce value option
* Split functionality into Datepicker and Calendar widget
* Fix item highlighting when month is changed
* Make use of uniqueId method
* Improvements for $.date
* Add min and max option
* Fix some minor style issues for multi calendar support
* Fix most demos
* Fixed and improved existing datepicker unit tests
* Improved localization, added localization unit tests.
General reminder: `external/date.js` is NOT an external library. It's not hosted (neither version-controlled) anywhere except in here. So, we may need to either make it an external library or move it inside `ui/`.