Dutch translation and i18n normalization/fixes

Hi sweeties,

This contains a 100% completed dutch translation as well as several i18n normalizations and fixes. 

## Internationalisation edits
- **context** added context strings for translations. For example review is ambiguous in dutch, plural form of hour is not used with cooking time and so forth. 
- **plural forms** also replaced the ugly php plural form check, to enable plural forms for languages such as Arabic and Slavic languages.
- **url translation** all URLs are now also translated, instead of just one out of two, when they appeared twice.
- **concatenation removal** removed the partial strings and replaced with formatted equivalents. Italian/Spanish translation need approx 10 updates. I did what I could with my knowledge of both.

##### Small changes
- Productor should have been producer
- Created a schema.pot (so no need to re-generate from wordpress plugin site while developing)
- Updated de Italian and Spanish po's
- Added translation for some untranslated strings
- Read the [commit](https://github.com/Derkje-J/schema-creator/commit/da5f0885801ca568d44dc0174d70e104884ba160) comment about casefix. 

## Auto Draft
I also updated the `save_` function to mimic the WordPress codebase. 

##### References
auto-draft commit: 6fa1b909dd4232bcbb28e2008ee1fa277a2b8a48
master head on request: a4eda2398caf6147855bec084d7dae893c1c109a