added method to determine distance between points

should be valuable to know what distance between two points is. Also
added testing

point.go

@@ -1,5 +1,11 @@
 package rect
 
+import "math"
+
 type Point struct {
 	X, Y float64
 }
+
+func distance(p1, p2 Point) float64 {
+	return math.Sqrt(math.Pow((p1.X-p2.X), 2) + math.Pow((p1.Y-p2.Y), 2))
+}