changed permission on func and added maximum error

made sure that not all functions were public and changed some to private
that need not be exposed.

also added in maximum error to deal with floating point inaccuracies

triangle.go

@@ -2,6 +2,6 @@ package rect
 
 import "math"
 
-func SizeTriangle(p1, p2, p3 Point) float64 {
+func sizeTriangle(p1, p2, p3 Point) float64 {
 	return math.Abs((p1.X*p2.Y + p2.X*p3.Y + p3.X*p1.Y - p1.Y*p2.X - p2.Y*p3.X - p3.Y*p1.X) / 2)
 }