Read draft from $schema

[yesennes]

When not provided a draft in the parameters, if a$schema field is present in schema, it should use that for drafts.

For instance
new Validator({"$schema": "http://json-schema.org/draft-07/schema#","type":"object"}).draft should be '7'

This is described in: http://json-schema.org/understanding-json-schema/reference/schema.html

I can put together a merge request for this.

[jdanyow]

that would be great, thanks!