add error handling in JSONDataHandler (#28) 

added unit tests for issues (#21) * added unit tests for issues * resolve issues * add tests for unmarshal_from_json_map * rename argument for clarity

update readme with new options 

15 support capturing nested unknown fields (#17) * implement JSONDataHandler support * implement JSONDataHandler support * add github stats badges

fix parse error on nested unknown fields (#14) 

9 unmarshaling doesnt work for composed types (#10) * add support for embedded fields * add unmarshal_from_json_map_test for embedding

refine api and docs according to internal reviews (#4) refine api and docs according to internal reviews