added in moveTo, updatePosition, and moveHorse function

moveTo takes an xy and an object and makes the object move towards the
xy at the objects speed. moveHorse moves the mustangs speed to move
towards whatever y-value is set for its track in the variable
trackYValues. updatePosition changes an objects (x,y)’s based on the
objects x,y speeds. both moveTo and updatePosition must be called every
update.