Opaque namespaces (at least "jar" URIs) do not work properly

[fge]

Test file: https://github.com/fge/json-schema-validator/blob/master/src/test/java/org/eel/kitchen/jsonschema/other/JarNamespaceValidationTest.java

Failing test: callingSchemaViaJarURINamespaceWorks() (except, it doesn't)

The test which breaks is due to a difference in behavior between:

factory = JsonSchemaFactory.defaultFactory();
schema = factory.fromURI("jar:/foo.jar!/a/b.json");

and:

factory = new JsonSchemaFactory.Builder()
    .setNamespace("jar:/foo.jar!/a/").build();
schema = factory.fromURI("b.json");

While the first solution will use the base URIDownloader and succeed, the second
one will fail since SchemaRegistry does not use JsonRef, which has a hack for
jar relative resolution, but URI resolution -- and resolving any URI against an
opaque URI, which "jar" URIs are, leads to the URI given as an argument.

First highlighted in issue #19.

[fge]

Done. Right now, only jar URIs are really supported, but due to JsonRef refactoring, this can easily be extended to other opaque URI schemes should the need arise.