Assume `build.rs` in the same directory as `Cargo.toml` is a build script (unless explicitly told not to)
So, in May I posted a question in the #cargo IRC room: https://botbot.me/mozilla/cargo/2016-05-26/?msg=66770068&page=1
This PR does what was discussed there. If `cargo` sees a `build.rs` in the same directory as the `Cargo.toml`, it will assume `build.rs` is a build script unless `package.build = false`. Just for completeness I also made `build = true` mean the same as `build = "build.rs"` but I'm not sure if that is okay or not.