Add support for json schema draft 4, 6 and 7

[rajpra8]

I saw that comment that because of lack of community support, newer version of json schema is not going to get supported. While going through latest specs, I noticed few breaking changes but I was able to make small tweaks to at least to get the schema written in version 7 be parsed by the code. Let me know if there is any work already in progress to add support for newer versions. I would love to join the effort.