cleaned up cmake file a little
Added a link to dlib on github
Added image_traits specializations for const types because it makes
certain compiler errors you get when accidentally trying to mutate
a const image a little easier to understand.
Fixed a bug in extract_image_chips() and get_mapping_to_chip() that caused
incorrect outputs when the requested chip stretched the image unevenly
vertically or horizontally.  This is because we used the best similarity
transform rather than affine transform between the image and the output chip.