add NewTube function (#45)

This allows creating *Tube values in a similar way a *TubeSet is created

conn.go

@@ -40,7 +40,7 @@ var (
 func NewConn(conn io.ReadWriteCloser) *Conn {
 	c := new(Conn)
 	c.c = textproto.NewConn(conn)
-	c.Tube = Tube{c, "default"}
+	c.Tube = *NewTube(c, "default")
 	c.TubeSet = *NewTubeSet(c, "default")
 	c.used = "default"
 	c.watched = map[string]bool{"default": true}

conn_test.go

@@ -8,7 +8,7 @@ import (
 func TestNameTooLong(t *testing.T) {
 	c := NewConn(mock("", ""))
 
-	tube := Tube{c, string(make([]byte, 201))}
+	tube := NewTube(c, string(make([]byte, 201)))
 	_, err := tube.Put([]byte("foo"), 0, 0, 0)
 	if e, ok := err.(NameError); !ok || e.Err != ErrTooLong {
 		t.Fatal(err)
@@ -21,7 +21,7 @@ func TestNameTooLong(t *testing.T) {
 func TestNameEmpty(t *testing.T) {
 	c := NewConn(mock("", ""))
 
-	tube := Tube{c, ""}
+	tube := NewTube(c, "")
 	_, err := tube.Put([]byte("foo"), 0, 0, 0)
 	if e, ok := err.(NameError); !ok || e.Err != ErrEmpty {
 		t.Fatal(err)
@@ -34,7 +34,7 @@ func TestNameEmpty(t *testing.T) {
 func TestNameBadChar(t *testing.T) {
 	c := NewConn(mock("", ""))
 
-	tube := Tube{c, "*"}
+	tube := NewTube(c, "*")
 	_, err := tube.Put([]byte("foo"), 0, 0, 0)
 	if e, ok := err.(NameError); !ok || e.Err != ErrBadChar {
 		t.Fatal(err)
@@ -61,7 +61,7 @@ func TestUse(t *testing.T) {
 		"use foo\r\nput 0 0 0 5\r\nhello\r\n",
 		"USING foo\r\nINSERTED 1\r\n",
 	))
-	tube := Tube{c, "foo"}
+	tube := NewTube(c, "foo")
 	id, err := tube.Put([]byte("hello"), 0, 0, 0)
 	if err != nil {
 		t.Fatal(err)

example_test.go

@@ -36,7 +36,7 @@ func Example_put() {
 }
 
 func Example_putOtherTube() {
-	tube := &beanstalk.Tube{Conn: conn, Name: "mytube"}
+	tube := beanstalk.NewTube(conn, "mytube")
 	id, err := tube.Put([]byte("myjob"), 1, 0, time.Minute)
 	if err != nil {
 		panic(err)

tube.go

@@ -11,6 +11,11 @@ type Tube struct {
 	Name string
 }
 
+// NewTube returns a new Tube representing the given name.
+func NewTube(c *Conn, name string) *Tube {
+	return &Tube{c, name}
+}
+
 // Put puts a job into tube t with priority pri and TTR ttr, and returns
 // the id of the newly-created job. If delay is nonzero, the server will
 // wait the given amount of time after returning to the client and before