Extend writemime to work with ColorValues besides RGB (fixes #130)
imedge/gradient operator updates

* Refactor imedge into imgradientxy, imgradient
* Added additional gradient operators from Ando Shigeru
  (Consistent Gradient Operators, PAMI Vol 22 No 3, March 2000)
* Add versions of sqrt, tan2 which return Images with Image input
* Changed Sobel, Prewitt operators to give gradient instead
  of negative gradient;  Also changed the order of arguments
  to return horizontal filter, vertical filter (Might
  need to be reverted)
* Added separable approximations of shigeru4 and shigeru5 filters
* Generalized imgradient functions to >2D