added i18n files for nl

copied files from the english version for a proof of concept
this works so we can now begin to translate the complete files
Merge remote branch 'upstream/master'
Merge remote branch 'upstream/master'
added dutch translations
translated simple_form
changed en: tag to nl: for dutch translation

changed wording for invalid email address
Merge pull request #3 from cygnus6/master

dutch translation