Added setting feature of FPS in cap_libv4l

Some time ago I saw this post related with the setting feature of the FramePerSecond property in v4l2 cameras (http://answers.opencv.org/question/7353/how-do-i-adjust-fps-for-a-v4l2-camera/)

There were suggested to include this patch in the OpenCV open source project, but It wasn't done until now. Because of I need this piece of code for some personal projects, I took the patch written in the previous post and now I'm sending this pull request :)

I can confirm that this piece of code runs correctly with the following cameras:
- Logitech C525
- Logitech C920
- Logitech Webcam Pro 9000