To Line class added method cropToImageEdges.

Crop the line if line's endpoints lies outside of image.
Test added for Line class method cropToImageEdges.
Corrected cropToImageEdges().

Added sotring for pt1 and pt2
Corrected bresenham_line method in Image class.

If endpoints of line lies outside of image,than line cropped to edges of
image. In old version endpoints cropped without saving slope of the
line.
Fix bug in cropToImageEdges() when there might appear more than 2
points.

It was due to rounding endpoints before checking that they are inside
image. Now they rounded after checking.

Fix bug with floating point.