Support acquisition information in LSL Gaze outlet

[papr]

The Gaze Meta Data format defines the following fields to hold information about the acquisition device:

<acquisition>        # information about the acquisition system 
  <manufacturer>     # manufacturer of the device
  <model>            # model name of the device
  <serial_number>    # serial number of the device
</acquisition> 

The new Pupil Labs Realtime API exposes this information via the /status endpoint.

To add the information, stream_info.desc() requires corresponding XMLElement child objects.