Add support for different styled date pickers on a single page

Added `base_css_class` setting to allow the modification of the css class of
the outermost container (defaults to `Zebra_DatePicker`, so existing users
has nothing to modify in their code). 

Can be very useful in single page applications (for example I used it to create a normal sized datepicker for inputs and a slightly different calendar-style, always visible one with huge font and cell sizes).