update Portuguese localization

All i18n strings included in the German l10n are now translated to
Portuguese. The comments in the file have also been partially
translated (where it made sense). The lang-pt.js and lang-de.js now
have parallel structures, since this makes it much easier to check for
missing translations.
Commit changes by Jens? I don't get it.
Merge pull request #2 from MMSequeira/master

Updated Portuguese l10n.