[DEV-257] update api

- Key now comes before value
- json_set() modifies it's gives result var
- If the result var is not initialized it is set to '{}'
- Removed tests/ so I don't have to update them. Tests were updated elsewhere.
[DEV-257] Update docs
[DEV-257] Don't change param order for json_get_field
Merge pull request #7 from flowroute/set_api

[DEV-257] change api