Simplify Codec and split responsibilities

Break Codec into two general purpose interfaces, Encoder and Decoder,
and move parameter codec responsibilities to ParameterCodec.

Make unversioned types explicit when registering - these types go
through conversion without modification.

Switch to use "__internal" instead of "" to represent the internal
version. Future commits will also add group defaulting (so that "" is
expanded internally into a known group version, and only cleared during
set).

For embedded types like runtime.Object -> runtime.RawExtension, put the
responsibility on the caller of Decode/Encode to handle transformation
into destination serialization. Future commits will expand RawExtension
and Unknown to accept a content encoding as well as bytes.

Make Unknown a bit more powerful and use it to carry unrecognized types.

pkg/runtime/unstructured_test.go

@@ -42,7 +42,7 @@ func TestDecodeUnstructured(t *testing.T) {
 	if pod, ok := pl.Items[1].(*runtime.Unstructured); !ok || pod.Object["kind"] != "Pod" || pod.Object["metadata"].(map[string]interface{})["name"] != "test" {
 		t.Errorf("object not converted: %#v", pl.Items[1])
 	}
-	if _, ok := pl.Items[2].(*runtime.Unknown); !ok {
-		t.Errorf("object should not have been converted: %#v", pl.Items[2])
+	if pod, ok := pl.Items[2].(*runtime.Unstructured); !ok || pod.Object["kind"] != "Pod" || pod.Object["metadata"].(map[string]interface{})["name"] != "test" {
+		t.Errorf("object not converted: %#v", pl.Items[2])
 	}
 }