Options to limit the number of item displayed in a views field

Right now, if you add a Node Referrer field to a View, you can choose if the values returned by nodereferrer_view_handler_field::referrers should be displayed as a list or comma separated, but you don't have any control on the number of items that are displayed.

Such a setting can be usefull if you only want the field to display the last node that referenced a given node, instead of all of them.

Attached patch adds such a setting in nodereferrer_view_handler_field::options_form, and uses it in nodereferrer_view_handler_field::render to limit the displayed result.

Another approach would be to pass the maximum number of elements to retrieve all the way down to nodereferrer_referrers and use it as a LIMIT in the SQL-query: that would be a bit more efficient, but would make the patch a lot more invasive.