Add Hough transform

Implemented the Hough Transform sample, adding the input image from the Rosetta website. Rather than use PNG and introduce another dependency, I converted it to PGM and wrote simple load/save support in Rust.  Compares exactly with the output of the Go version (on which it was based).
on main you need to add

    #[cfg(not(test))]

to avoid this https://travis-ci.org/Hoverbear/rust-rosetta/builds/38453682#L358