Datepicker Should Allow Removal of ui-datepicker-current Button

[jhamburger]

Currently when showButtonPanel is set to true automatically there are two buttons(ui-datepicker-current and ui-datepicker-close). The ui-datepicker-current button returns to the current selected date however the button can be ambiguous and is not very useful. Thus we need an option to control the showing of this button, maybe a showCurrentButton option with a default of true.