added support for setup without calibrationFile to track and draw 2d

if nothing is passed to camParams on ofxAruco::setup() then camParams.isValid() will be false and 3d pose estimation won't be performed by Aruco, which is fine if you wan't to work in 2d only. Also, refactored ofxAruco::draw() method to be:

void ofxAruco::draw()
{
  if ( camParams.isValid() ) 
    draw3d();
  else draw2d();
}