Merge from branch - #70

This uses a lot of images because generating tests for these is hard. The
image files are all very small (<5kb) so it shouldn't make downloading
the repo significantly slower.

This algorithm can be tricky to get right so I wanted to have a very
exhaustive test suite.

* pygorithm/geometry/extrapolated_intersection.py - add more helper
functions
* pygorithm/geometry/polygon2.py - fix fuzzing test sometimes failing
* tests/test_geometry.py - add regression test for polygon, add tests for
first two functions of extrapolated_intersection
+ imgs/test_geometry/test_extrapolated_intersection - images that help
visualize what the tests are doing. the naming convention is explained
in test_geometry.TestExtrapolatedIntersection docstring

Add tests and images for calculate_one_moving_and_one_stationary in
extrapolated intersection. This is at the same rate of 16 tests per func
(16 images and 16 .grf files)

* pygorithm/geometry/extrapolated_intersection.py - change to invalid
return values so the tests fail as expected

* tests/test_geometry.py - add tests for an additional function
+ imgs/test_geometry/test_extrapolated_intersection/(many) - test imgs.

This is a sample of what the images look like using matplotlib instead of
Graph.

+ imgs/test_geometry/test_extrapolated_intersection/
	aa01_test_point_line_no_intr.py - matplotlib to generate:

+ imgs/test_geometry/test_extrapolated_intersection/out/
	aa01_test_point_line_no_intr_2.png - matplotlib generated file

Replace all the graph files and generated images with matplotlib files and
generated images. This is from @IanDoarns suggestion on
#69.

* imgs/test_geometry/test_extrapolated_intersection/(many) - replace with
	matplotlib files

* tests/test_geometry.py - update documentation to remove references to
	Graph files (in favor of matplotlib)