Add jpeg direct publishing without decoding

This patch add an image_encoding = "jpeg" to gscam, to create a gstreamer pipeline just connecting to one of these webcams producing mjpeg (e.g. axis camera) but without decoding the stream. On some embedded system, this is useful to log the video stream as jpegs but only decompress it on request. 

The behaviour is roughly the same as the axis_camera driver, but without the huge latency sometimes introduced by python. 

Without the parameter "image_encoding == jpeg", the patch does not change anything to the code.