Added possibility to stop streaming

added function headtrackr.Tracker.stopStream() to stop a created stream,
also added stream object as the property headtrackr.Tracker.stream
Added possibility to use video "as is"

Added the parameter setupVideo to init, to allow possibility to do
headtracking on video element already set up, i.e. without headtrackr
trying to setup mediastream from webcam.
Fixed bugs from last 2 commits