Resolve relative schema URIs

[gertvv]

From a schema at classpath:/com/example/outer.schema.json, relative $ref should resolve from that base URI. For example, "$ref": "inner.schema.json" should resolve to classpath:/com/example/inner.schema.json. Instead, the following error is raised:

java.lang.NullPointerException: Cannot invoke "java.net.URI.toString()" because "referencingFileURL" is null

This would make the schema files publishable as-is on the web.

Follow-up to #29. Test case here.