Added the ability to populate attributes with existing translations via directive by adding a new convention of referencing an existing translation with i18n.t.xxxxx where xxxxx is the translation you wish to use in a new translation.  A real world example of this, is if you have a translation for the word password and have error handling for the when that field isn't filled out.  Instead of translating the word 'password' multiple times your translation can now reference it.  Updated doc and tests to reflect changes.