Add an option to configure the class used to wrap the nested fields. (brought up in issue #178)

My solution was to use the `html_options` parameter of #link_to_remove_association.
If it contains a `wrapper_class` key, it saves that key in the link's data-wrapper-class attribute,
and then uses it in the JS, when trying to remove an item.
Merge pull request #180 from yoavmatchulsky/nested-fields

Add an option to configure the class used to wrap the nested fields. (br...