Changes the json module to use the Jansson library with the json path patches.

TODO: update docs/README
Big changes for the json module.

- Fix up errors in module interface input.
- Add an array sizing script function, json_array_size.
- Able to return sub objects from partial paths.
- General fixes.
Free string returned from json_dumps
Unit tests
Fix null unit test
Merges json_path from jansson into module.
Add back script function json_get_field for backwards compatibility.
Update documentation.
Merge pull request #1 from flowroute/jansson

Jansson