Configurable check before ajax

Hi,

in some cases -such as file upload- wich is done via AJAX requieres the form validation and there isn't a easy way to exclude some inputs from this check.

I've developed a path to get a textarea in the configuration form where can be set the ID of input elements to bypass the "before AJAX" check.

I think that functionality can be usefull for most usercases, so I contribute it to be considered.

[I know that can be added the class 'cancel' to input element, but for widgets provided for contributed modules is so hard task, and by this way the clientside_validation provides a more consistent functionality]

thanks for your amazing work!