Implement NewConn

Step 1 for fixing #18.

conn.go

@@ -104,13 +104,23 @@ func SystemBus() (conn *Conn, err error) {
 
 // Dial establishes a new connection to the message bus specified by address.
 func Dial(address string) (*Conn, error) {
-	var err error
-	conn := new(Conn)
-	conn.transport, err = getTransport(address)
+	tr, err := getTransport(address)
 	if err != nil {
 		return nil, err
 	}
-	if err = conn.auth(); err != nil {
+	return newConn(tr)
+}
+
+// NewConn creates a new *Conn from an already established connection.
+func NewConn(conn io.ReadWriteCloser) (*Conn, error) {
+	return newConn(genericTransport{conn})
+}
+
+// newConn creates a new *Conn from a transport.
+func newConn(tr transport) (*Conn, error) {
+	conn := new(Conn)
+	conn.transport = tr
+	if err := conn.auth(); err != nil {
 		conn.transport.Close()
 		return nil, err
 	}
@@ -123,7 +133,7 @@ func Dial(address string) (*Conn, error) {
 	go conn.inWorker()
 	go conn.outWorker()
 	go conn.serials()
-	if err = conn.hello(); err != nil {
+	if err := conn.hello(); err != nil {
 		conn.transport.Close()
 		return nil, err
 	}

transport_generic.go

@@ -0,0 +1,34 @@
+package dbus
+
+import (
+	"errors"
+	"io"
+)
+
+type genericTransport struct {
+	io.ReadWriteCloser
+}
+
+func (t genericTransport) SendNullByte() error {
+	_, err := t.Write([]byte{0})
+	return err
+}
+
+func (t genericTransport) SupportsUnixFDs() bool {
+	return false
+}
+
+func (t genericTransport) EnableUnixFDs() {}
+
+func (t genericTransport) ReadMessage() (*Message, error) {
+	return DecodeMessage(t)
+}
+
+func (t genericTransport) SendMessage(msg *Message) error {
+	for _, v := range msg.Body {
+		if _, ok := v.(UnixFD); ok {
+			return errors.New("unix fd passing not enabled")
+		}
+	}
+	return msg.EncodeTo(t)
+}