Better error handling, new commands.

The code is no safer.
Added Stor(), Rename(), NoOp()
Improved Retr()
Added client tests (requires a ftp server on localhost)

client_test.go

@@ -0,0 +1,55 @@
+package ftp
+
+import (
+	"bytes"
+	"io/ioutil"
+	"testing"
+)
+
+const (
+	testData = "Just some text"
+)
+
+func TestConn(t *testing.T) {
+	c, err := ConnectAnonymous("localhost:21")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	err = c.NoOp()
+	if err != nil {
+		t.Error(err)
+	}
+
+	data := bytes.NewBufferString(testData)
+	err = c.Stor("test", data)
+	if err != nil {
+		t.Error(err)
+	}
+
+	_, err = c.List(".")
+	if err != nil {
+		t.Error(err)
+	}
+
+	err = c.Rename("test", "tset")
+	if err != nil {
+		t.Error(err)
+	}
+
+	r, err := c.Retr("tset")
+	if err != nil {
+		t.Error(err)
+	} else {
+		buf, err := ioutil.ReadAll(r)
+		if err != nil {
+			t.Error(err)
+		}
+		if string(buf) != testData {
+			t.Errorf("'%s'", buf)
+		}
+		r.Close()
+	}
+
+	c.Quit()
+}