compute projective transformation

fix Travis build

new tests

fix video script

video script

Merge pull request #5 from SebKuzminsky/cleanups

manual_lines._lines(): rename `x` to `mid_line`

manual_lines._lines(): refactor to simplify

manual_lines._lines(): comment the n=0 case

manual_lines._lines(): break up the n=0 case, for legibility

manual_lines.half_line(): refactor for simplicity

manual_lines.half_line(): add code comments

manual_lines.py: add some unit tests

add unit test runner script, add to travis

geometry.line(): add arguments and return value to doc string