calibrate camera using opencv tool
funcs to convert between calibrated and uncalibrated image points
use calibrated points in pose estimation
try calibrating a few more times

Notice how the first 2 diagonal entries in the camera matrix given
by opencv vary a lot... Not sure if it's a case of ambiguity, or if
the calibration tool is just acting up.
tweak camera matrix so that our height measurements are approx correct
clear corners image if no corners detected
CaptureFPS: display captured images optionally
CaptureFPS: exit gracefully on ctrl+c

(Otherwise, it doesn't close the camera properly, and will refuse
to run until the camera is unplugged and plugged in again.)
CaptureFPS: show datetime when frame was captured
Merge branch 'master' of github.com:cberzan/drones-287