Issue #4: StereoCalibrator class now inherits from Calibrator class.
Additional changes:
-modified way of constructing vectors of object and image points in Calibrator class
-minor changes in some Calibrator class methods in order to be compatible also with StereoCalibrator
-added flags (and setters for them) in Calibrator class(and also for StereoCalibrator) to display corners and show undistorted images
-dispatched some variables from execute method in StereoCalibrator class into class fields
-added flag bouguetsMethod and setter for it which defines method of calibration

Fixes:
-precomputing maps for remaps. It was bugged since my last commit (sorry for that)