Add `mocha` and `should` to `package.json`

These packages are used in testing, so it makes sense to list them as
developmental dependencies.
Use `url.resolve` to concatenate urls

The documentation for `url.resolve` says:

>   Take a base URL, and a href URL, and resolve them as a browser would
>   for an anchor tag.

I believe this is the behavior we want. It will insert slashes where we
want them, and keep from putting them where we don't.