Translate missing spanish strings

Translate missing strings from `es.yml`.

Also, a question. I wanted to add translation for the model name, something like this:

    es:
      activerecord:
        models:
          admin_user: 'Administrador'

It's ok to add keys/strings to a locale file that aren't present in `en.yml`? I ask because that file is usually considered the canonical list of strings to translate, but I noticed some Formtastic strings here in
`es.yml`