Make FIELD_ERROR_PROC configurable

Minor change on field_error_proc

Make FIELD_ERROR_PROC configurable, activerecord helper to active model helper
Setting custom field_error_proc test
Ensure that field error proc is swapped by simple_form_for
Remove unnecessary test
Remove field_error_proc doc from initializer
Merge pull request #773 from dfens/master

Make FIELD_ERROR_PROC configurable