Fix line algorithm implementation

[byronwasti]

This implements the Bresenham's line algorithm from the wikipedia page with some slight modifications.

fixes #11

[jamwaffles]

Thanks for taking a crack at this! A couple of things:

1. Can you rustfmt your code please?

2. It would be good to have at least a primitive unit test for this code, just to catch any glaring problems. I wrote a crappy one for my fix which passes when pasted into this PR. It's not the greatest test, but it should hopefully catch any regressions in the future.

[byronwasti]

I've added tests for each octant. I'm not sure if its the best way to test things, but it should catch regressions.

[jamwaffles]

Looks good! I like your approach to the testing, much better than mine as it actually checks the line. Thanks for fixing the lines!