Differentiate between different drafts and metaschema validation

[johandorland]

Draft-06 and draft-07 were implemented in a way that the API was unchanged, as it was quite limited. New features introduced in these drafts were basically tagged on the draft-04 functionality, without providing an API to distinguish between all these different drafts, creating some sort of hybrid draft.

With the recent addition of SchemaLoader we can provide a way of distinguishing between different drafts. This PR adds support for:

• Explicit support for draft-04, draft-06, draft-07 and a hybrid mode.
• Automatic detection of different drafts using $schema
• Draft cross-referencing support. A draft-04 schema can reference a draft-07 schema and both schemas will be parsed according to their appropriate draft version
• Validation of the input schema by validating them against the used draft's metaschema, or in case of a custom $schema, that particular custom schema.
• Local caching of metaschemas, so metaschema validation can be done offline.