[VA] {refactoring}  I19 enemy behavior

#19 enemies no longer chase players; instead they look at the player; if
timer goes to zero -> gameover
[VA] {refactoring} MathSneakquest.vectorToDegrees

changed Math.atan to Math.atan2 (converts rectangular coordinates to
polar coordinates)
[VA] {issue} I40 common update method

closes #40
player and enemy use entity.update; only difference is rotation handling
[VA] {refactoring} I19 new unit tests for enemy

#19 added new unit tests for enemy class