Implement oneof support.

This includes the code generation changes,
and the infrastructure to wire it up to the encode/decode machinery.

The overall API changes are these:
  - oneofs in a message are replaced by a single interface field
  - each field in a oneof gets a distinguished type that satisfies
    the corresponding interface
  - a type switch may be used to distinguish between oneof fields

Fixes #29.
diff --git a/proto/equal_test.go b/proto/equal_test.go
index b322f65..0e0db8a 100644
--- a/proto/equal_test.go
+++ b/proto/equal_test.go
@@ -180,6 +180,24 @@
 		&pb.MessageWithMap{NameMapping: map[int32]string{1: "Rob"}},
 		false,
 	},
+	{
+		"oneof same",
+		&pb.Communique{Union: &pb.Communique_Number{41}},
+		&pb.Communique{Union: &pb.Communique_Number{41}},
+		true,
+	},
+	{
+		"oneof one nil",
+		&pb.Communique{Union: &pb.Communique_Number{41}},
+		&pb.Communique{},
+		false,
+	},
+	{
+		"oneof different",
+		&pb.Communique{Union: &pb.Communique_Number{41}},
+		&pb.Communique{Union: &pb.Communique_Name{"Bobby Tables"}},
+		false,
+	},
 }
 
 func TestEqual(t *testing.T) {