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/text_parser_test.go b/proto/text_parser_test.go
index 0754b26..a2a9604 100644
--- a/proto/text_parser_test.go
+++ b/proto/text_parser_test.go
@@ -486,6 +486,18 @@
 	}
 }
 
+func TestOneofParsing(t *testing.T) {
+	const in = `name:"Shrek"`
+	m := new(Communique)
+	want := &Communique{Union: &Communique_Name{"Shrek"}}
+	if err := UnmarshalText(in, m); err != nil {
+		t.Fatal(err)
+	}
+	if !Equal(m, want) {
+		t.Errorf("\n got %v\nwant %v", m, want)
+	}
+}
+
 var benchInput string
 
 func init() {