Add support for resource URIs (<http://example.org/class>) to Graph->isA...

... resp. Resource->isA methods

Currently for checking a type of a resource requires its namespace to be registered:

    RdfNamespace::set('ex', 'http://example.org/');
    $resource->isA("ex:class")

with this pull request the `isA` method of the `Graph` and `Resource` classes will also support complete URIs

    $resource->isA("<http://example.org/class")