Move rs_pbrt to a proper binary

[abusch]

At the moment the CLI rs_pbrt is under examples, but as it's the main interface I think it should be a proper binary and be moved under src/bin/rs_pbrt.rs. This way it can be invoked by simply doing cargo run which most users would expect.

[wahn]

OK. I'm not sure how to do this in Cargo.toml, and if it has an effect on the CI with Travis, but lets do it. Go ahead if you know what has to be done ... I will update the Makefile if neccessary.