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
* Some fixes for the unit tests
* Fix some minor style issues for multi calendar support
I've implemented this but I'm not quite sure about it. Perhaps we can simplify this by moving the setting of $.ui.calendar.prototype.options to the _create method and just use $.extend?

Please take a look.