Added setting so control audio volume

You can now pass a video setting on initialization:

        $('.video-container').videoBG({
            mp4:'./src/media/bg.mp4',
            ogv:'./src/media/bg.ogv',
            webm:'./src/media/bg.webm',
            poster:'./src/media/bg.png',
            scale:true,
            zIndex:0,
            volume: 0 // values 0 to 1
        })