add xpath selector feature for select2 container

I have some select2 without label in my forms. So I needed to find their container by id. These ids are sometimes quite complex (eg. : s2id_product_prices_attributes_1369207263564_currency_id), so call them with xPath syntax is really usefull in these cases.

With my changes, it's now possible to use this syntax : 
select2("Dropdown Text", xpath: ".//path_to//the_dropdown_container")

this patch can easily be used with my previous one (select2-multiple)