Fix rotation of filter kernel (2d) and add tests

From https://github.com/GunnarFarneback/julia/commit/62e58c6154d5d6178bd06d5dce466a3ba51b06a5
Add array padding and replace border handling in imfilter with it

From https://github.com/GunnarFarneback/julia/commit/4711b9356cd658c7342cab1a44a5a35b35cc73ab
Improve colorspace conversion
Permit forcing load of particular file formats
Major enhancement of documentation