Added full i18n for plugin, plus default pot file, plus German translations

I added full internationalization (i18n) syntax to the whole plugin!

1) added i18n gettext syntax (Text domain: 'woo-product-importer' --- plugin slug is always recommended for that :)

2) added default .pot/.po files for translators

3) added full German translations based on the above changes/additions.

Added "/languages/" folder to host the translations.

Thanks, Dave from Germany :)