Implement id parsing in conjuction with $ref fixes

[johandorland]

This commit adds support for the id keyword and rewrites part of the code that deals with schema references. In conjuction with xeipuuv/gojsonreference#8 this fixes issues #134, #143, #168, #170, which were all related to schema references in one way or another.

[sqs]

Thank you! I am not the maintainer, but I tried this out and it works for me.

It solves the problem I encountered where I wanted to avoid accessing the network to resolve referenced JSON schemas and instead provide them as strings. There may be an even better way to do it, but with this PR, I was able to use code like (simplifed):

type jsonLoader struct {
	gojsonschema.JSONLoader
}

func (l jsonLoader) LoaderFactory() gojsonschema.JSONLoaderFactory {
	return &jsonLoaderFactory{}
}

type jsonLoaderFactory struct {
	schemas map[string]string
}

func (f jsonLoaderFactory) New(source string) gojsonschema.JSONLoader {
	switch source {
	case "https://sourcegraph.com/v1/settings.schema.json":
		return gojsonschema.NewStringLoader(schema.SettingsSchemaJSON)
	case "https://sourcegraph.com/v1/site.schema.json":
		return gojsonschema.NewStringLoader(schema.SiteSchemaJSON)
	}
	return nil
}

And then use it in Validate like so:

	return gojsonschema.Validate(
		jsonLoader{gojsonschema.NewBytesLoader(schema)},
		gojsonschema.NewBytesLoader(input),
	)

The code above works with this PR and not on master.