Dramatically improved i18n stuff, updated German translations etc. ...

1) added optional WP_LANG_DIR location (that location is also looked at, avoids overridden lang files on plugin updates - optional if user wants to use that location)

2) unified all textdomains to one and only 'ninja-forms'

3) added dozens of missing i18n/Gettext syntax (I guess I catched them all, but there may be more?)

4) expanded and updated default .po/.pot file for translators

5) completed and improved packaged German translations (informal version), all in light of above tweaks & additions

Thanks, Dave from Germany :)