Dutch translation, i18n normalization/fixes, Settings API3, Extensions

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.

## Settings API
Updated the class so that it uses the new Settings API. This means that the settings pages look like those from WordPress. This should be the new defacto standard for all your settings. 

## Extensions
Added hooks and filters so that the base class functionality can easily be extended. Currently supports tooltips, settings and metabox action hooks and filters. Little bit of documentation in the readme.

## Other 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. 
- Removed `readme.txt` and replaced with `readme.md` with markdown formatted contents. See my fork page.

## Auto Draft
I also updated the `save_` function to mimic the WordPress codebase. 

##### References
auto-draft commit: Derkje-J/schema-creator@6fa1b909dd4232bcbb28e2008ee1fa277a2b8a48
master head on request: Derkje-J/schema-creator@a4eda2398caf6147855bec084d7dae893c1c109a

##### Checklist for some final testting
- [x] creating 4 posts
- [x] setting all the permutations of the two post_meta settings
- [x] setup don't output css, leave the include options intact.
- [x] merge/update
- [x] see if all permutations still exist
- [x] see if the settings still says don't output css