Added a vertices getter to RegularPolygon

Assumes that the first vertex is on the X-axis
RegularPolygon's center property should default to PointZero
Fixed Transformation addition and subtraction

But it still only works for pure translations
Added bounds, min, max, and minmax properties to RegularPolygon
Added an edges property to RegularPolygon
Fixed handling of nil translation in Transformation#transform
Fixed composition of Transformations with nil translations
Fixed Point#clone

Override Vector#clone to force the return value to be a Point
Minor cleaning in Point specs
Added support for rotations in Transformation#transform
Point.zero can now optionally return a Point full of zeros

Pass a size argument to get a Point of a desired size. Otherwise it returns a PointZero.
Implemented RotationAngle composition
Added Transformation::Composition

A mechanism for composing transformations that doesn't force everything to be a matrix
Minor documentation cleaning in Point and Size
Added a has_rotation? property to Transformation and Composition
Fixed the Line#slope attribute

It was only partially implemented