Rename _history to _eye_detection_history.
Simplify eyes.py

* LeftEyeLocator.locate() now returns a boolean instead of raising an exception.
* _eye_detection_history now stores booleans instead of None and locations.
Introduced History class to manage length of histories.
Removing extra call to pointer.get_location() and moving updating
of pointer history next to updating eye detection history.
Making magic numbers and complicated operations more meaningful.