Added bindings to the Affine transformation functions.

The following built-in functions are now wrapped:

  gdTransformAffineCopy()
  gdTransformAffineGetImage()
  gdTransformAffineBoundingBox()
  gdAffineExpansion()
  gdAffineRectilinear()
  gdAffineEqual()
  gdTransformAffineBoundingBox()
  gdAffineIdentity()
  gdAffineScale()
  gdAffineRotate()
  gdAffineShearHorizontal()
  gdAffineShearVertical()
  gdAffineTranslate()

This change also defines class Affine, which wraps the actual
transformation matrix (array, actually).