[Extractor] Improve JsonEtractor

Add ability to parse json to extract only desired part
Merge pull request #14 from tyx/feature/new-json-extractor

[Extractor] Improve JsonEtractor
[Extractor] Small fix for JsonExtractor

To handle json files which gets only object
Merge pull request #15 from tyx/fix/json-extractor

[Extractor] Small fix for JsonExtractor