Allow customized text for new record link in has_many forms.

Overload the :new_record option, so that if a string is provided, it
will use the string for the new_record link text. Fallback to the
existing behavior if :new_record is not a string.
Merge pull request #2636 from clalimarmo/custom-has-many-new-record-link

Allow customized text for new record link in has_many forms.