add ability to pass optional parameters to content_tag

Previously, you couldn't pass options to the nested_fields_for
method expect :wrapper_tag and :legend. Now users can pass
a third option, :tag_options which accepts a hash of options
that can be passed to the content_tag method.
update README to document tag_options feature
change tag_options to wrapper_options and add functionality for accepting classes in an array
Merge pull request #13 from dustinbrownman/master

add tag_options option