Datepicker improvements

- A datepicker is about 200 bindings. On a page with many datepickers (20 in my use case) this amount of bindings has a real impact on performance.
    - I suggest to add a boolean lazyDom parameter to datepickerPopupConfig. When this parameter is true, the datepicker popup Dom and bindings are created on focus and deleted on blur.

- The datepicker popup root element (UL) has no attribute or class to allow jquery/angular.element selection
    - I suggest to add a 'datepicker-popup' class to this UL element