Added helper to simple_form

SimpleForm provides excellent handling of validation errors for any
attributes of a model. However, some applications set model errors that
are not matched to a specific form field. These are called “base
errors.” For example, a model might set an error like this: errors.add
:base, "Credit card declined". SimpleForm does not provide a helper to
display base errors; if your application sets base errors, you’ll need
to add the following view helper.