Better shot threading

[hbredin]

Thresholding the minimum number of matches is not good enough.
We might also want to use cv2.findHomography and RANSAC:
http://docs.opencv.org/2.4/modules/calib3d/doc/camera_calibration_and_3d_reconstruction.html?highlight=findhomography#findhomography

Documentation says:
Note that whenever an H matrix cannot be estimated, an empty one will be returned.

This "features" might be used as a second filtering step...