Minor cleanup of algorithms code.
Merge pull request #131 from kmsquire/alg_cleanup

Minor cleanup of algorithms code.
Fix #132
Fix #129
Return Image when imfiltering Image (fix #133)
imedge/gradient operator updates

* Refactor imedge into imgradientxy, imgradient
* Fix 2D gradient calculation
* Added additional gradient operators from Ando Shigeru
  (Consistent Gradient Operators, PAMI Vol 22 No 3, March 2000)
* Set shigeru3 as default gradient method for imgradient, imgradientxy
* 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
Gradient updates

* added magnitude, phase functions
* removed extra temporary creation from phase calculation
* added hypot function for Images (faster than sqrt(x.^2+y.^2))
* added checkerboard, diagonal gradient tests
Minor gradient updates

* Revert order change for filters
* Minor update to tests
Updated gradient function documentation
Gradient function fixes

* Add orientation function for returning the strongest edge orientation
* Make imedge return edge orientation instead of gradient direction
* Update docs, tests
Merge branch 'gradients' of github.com:kmsquire/Images.jl into gradients