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.
Fixed bug in return format of line.

Should return endpoints as tuple, not as list.
Merge branch 'extend_crop' of github.com:polart/SimpleCV into develop