added option 'appendWidgetTo' and fixed calculation of display position

Attaching the datepicker container always to the 'body' does not work as expected, if the input element is positioned in a scrollable parent container using 'overflow: auto'. The datepicker stays on his absolute position. I fixed the positioning by calculating the left and top values relative to the 'appendWidgetTo' container (default 'body'). Next fix will be the detection of scrollLeft, too (sorry, no time for now).