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.
has_many form code style update
remove old form code
add an overall description to the AA form builder
update the way has_many form fields are built

If a user clicks the create button in rapid succession, it's possible
for you to have multiple new records with the same identifier. This
change adopts the same identifying scheme that the form itself already
uses for child records: a numeric index.