Added cropToImageEdges method to Line class
Corrected method extendToImageEdges in Line class.
Tests added for cropToImageEdges and extendToImageEdges
Corrected bresengam_line method in ImageClass.

There was a bug with cropping lines. So if the line bigger than image
then it just set endpoints to image edges without saving slope of the
line.
Merge branch 'extend_crop' of github.com:polart/SimpleCV into polart-extend_crop
uhg standard