Add support for mutiple disc.

Some games require switch umd to continue.

e.g. Grisaia no Kajitsu: La Fruit de la Grisaia

Requires disc2.
![qq20131124125530](https://f.cloud.github.com/assets/3080636/1608064/c7d408ca-54c5-11e3-9992-c8258e4d4494.png)

Pause, switch umd.
![qq20131124125808](https://f.cloud.github.com/assets/3080636/1608065/f1729282-54c5-11e3-9956-531db23c510b.png)

Select disc2.
![qq20131124125827](https://f.cloud.github.com/assets/3080636/1608066/0079127e-54c6-11e3-8b8f-2988328bd1c8.png)

Continue
![qq20131124125903](https://f.cloud.github.com/assets/3080636/1608068/16067780-54c6-11e3-95f0-1ffc0681a627.png)

Ok.
![qq20131124130118](https://f.cloud.github.com/assets/3080636/1608069/2182efa8-54c6-11e3-9468-0b1afaa3804c.png)