Decoding doesn't work properly

Hey chai,

Thank you for the great tool! Encoding works fast and well!
However, decoding isn't really working properly.
I would like to encode as much text as possible and encountered issues, as soon as the text is more than 100 characters long.

You can try the following modified `hello.go` with the parameters (tested on Win10, x64):
```go
// Copyright 2014 . All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// +build ingore

// QR codes encoder.
package main

import (
"bytes"
"flag"
"fmt"
"image"
_ "image/jpeg"
_ "image/png"
"io/ioutil"
"log"
"os"
"path/filepath"
"strings"

"github.com/chai2010/qrcode"
)

var (
flagText = flag.String("text", "chaishushan@gmail.com", "Set text")
flagLevel = flag.String("n", "M", "Set QR encode level(L|M|Q|H), default is 'Q'.")
flagOutput = flag.String("o", "qrcode.png", "Set output filename (PNG only).")
)

func init() {
flag.Usage = func() {
fmt.Fprintf(os.Stderr, "Usage: %s [options] text\n", filepath.Base(os.Args[0]))
flag.PrintDefaults()
}
}

func main() {
flag.Parse()

filename := filepath.Clean(*flagOutput)
if filename == "" {
filename = "qr.png"
}
if !strings.HasSuffix(strings.ToLower(filename), ".png") {
filename += ".png"
}

level := qrcode.Q
switch strings.ToUpper(*flagLevel) {
case "L":
level = qrcode.L
case "M":
level = qrcode.M
case "Q":
level = qrcode.Q
case "H":
level = qrcode.H
}

code, err := qrcode.Encode(*flagText, level)
if err != nil {
log.Fatal(err)
}
err = ioutil.WriteFile(filename, code.PNG(), 0666)
if err != nil {
log.Fatal(err)
}
fmt.Println("Save as:", filename)

// Load file data
data, err := ioutil.ReadFile(filename)
if err != nil {
log.Fatal(err)
}

// Decode image
m, _, err := image.Decode(bytes.NewReader(data))
if err != nil {
log.Fatal(err)
}

// Decode QR Code
text, err := qrcode.Decode(m)
if err != nil {
log.Fatal(err)
}
fmt.Println(filename, ":", text)
}
```

Output

> C:\Users\JamesCullum\go\src\github.com\chai2010\qrcode>go run hello.go -text=uGS8wfili1eOUZfXBFFp8mmVyrOg7xROMF0tljABst6K1h3z5O4Fd3AA -n=L
> Save as: qrcode.png
> qrcode.png : uGS8wfili1eOUZfXBFFp8mmVyrOg7xROMF0tljABst6K1h3z5O4Fd3AA
>
> C:\Users\JamesCullum\go\src\github.com\chai2010\qrcode>go run hello.go -text=uGS8wfili1eOUZfXBFFp8mmVyrOg7xROMF0tljABst6K1h3z5O4Fd3AA1xBDsVGhHl2J2GGM5Xf1Br1tHRHpsVSvSxYehyF1EAZGloDxCQNgnv7omEkkPvC1Z8xZPCjob7YnHSclHQnOpVYo0PIS5z -n=L
> Save as: qrcode.png
> qrcode.png : uGS8wfili1eOUZfXBFL�`�ʼ����n𤞊�p��Ԓ���l�r�v�j�x��v��b���漞���t�d���z��r�r1tHRHpsVSvSxYehyF2�����Θ�������~�ʊ�֠��b�pഠ����n�̀��؀�̎༲�`���j�

In each case for more than 100 characters, the characters get encoded weirdly and are incorrect. For characters longer or higher error correction, the script crashes.