Minor: fix for ST2.1, changes. Added currentDateButton to picker

- Minor fix for ST2.1 in DateTimePicker:  after sequence: picker cancel(on
cancel is destroied), picker show; picker is not seen because
innerHtmlElement garbage. Added innerHtmlElement.destroy()

- Added currentDateButton to set/reset picker to current date

- To receive only one event I've deleted the line where fireEvent is done in Ext.ux.field.DateTimePicker.onPickerChange.