Added possibility to manually decide whether a check box is checked

I wanted the possibility to decide whether a check box is checked or not by passing the corresponding value (true or false) to the input. This now can be done by passing :input_html => { :checked => true/false }. Previously you only could check the input but not _uncheck_ it, e.g. when builder.object.send(:method) returns true and you want the corresponding check box unchecked

I hope that helps.

Jan