Add integration with JSON Field module

OK, then why not. I'm not familiar with JSON field module, so we need to investigate:
- if we need a new widget based on the current one, or can it be modified to support this other field type from JSON field module (without breaking backwards compatibility)
- for the formatters, it should be easier: maybe we don't need to json_decode a string during processing the output - if that field type gives us the JSON right away. I can't tell now, so this must be checked.
- also if that JSON field module defines multiple field types, we should consider if we want to support everything, or select a few.
- also we have to keep in mind that this should be an optional dependency right now. Existing code should continue to work.

Well, it was easier than I expected. Will be out in the latest release.

Automatically closed - issue fixed for 2 weeks with no activity.