Add new frame type and framer factory, support multiple image types. Add snapshot and imageserver.

[aamcrae]

Create a frame type that wraps webcam frames as a Go image type. Build a framer factory registration that allows new image types to be supported, based on the FourCC strings in V4L. Implement wrappers for jpeg, mjpeg, YUYV422 and RGB frames.

Build a snapshot package that captures individual frames from the webcam stream. Create a new examples/imageserver that demonstrates a web server serving jpeg/png/gif images from the webcam. imageserver also provides a controls interface allowing parameters to be set in V4L.