Join driver

Driver to join images from different streams into a single `pb::CameraMsg` structure. The idea is that we can use all our programs that get a single `-cam` parameter (e.g. vicalib, SensorViewer) without modifications.

Syntax:
`join://<driver1>&<driver2>&...&<driverN>`

Example to join images from a Kinect and a Kinect2 cameras:
`join://openni://&freenect2://`

Note that the `&` must be escaped in the command line.