feat(restish): properly handle binary download and use

What

This MR fix the way we handle external binaries such as Restish. It also fix some minor coding style issue.

Why

Because we hardcoded the link to the linux-x86_64 version of restish binary, so it did not work on Mac / Windows computers.

How

Use the platform module to have the correct URL, and throw a proper error when the url is wrong.

References

Closes #9 (closed)