Added support for selecting the audio capture device (fixes #220)

For a project I did I needed to be able to choose the audio capture device (i.e. external soundcard, microphone, webcam mic etc.) I implemented it and now I want to share it to extend the multimedia functionality of SFML. The code is tested and works. The only thing I'm not really sure about is the design. I welcome everybody to give me some feedback.
This is a fixes the issue #220.