Moves /developer.html to /developer/, and update links to resources

Re: #88, this moves `/developer.html` to `/developer/`. Note the trailing slash - as our server stands, that's not optional. This doesn't add a redirect, since the `/developer.html` link has been live for almost no time.

Also, this updates the resource links to add `../` to them, since this moves the page to a sub-path, e.g. `../js/main.js`). I'm not thrilled about that - changing this to use references from the root of the path (e.g. `/js/main.js`) make it stable and irrelevant what path you're referencing it from. But that would break out staging site, which runs from its own subpath, at `18f.github.io/18f-staging`, so I didn't make that change.