Adding support for `cleanurl: false` metadata

I have my site on Github Pages which serves up 404.html at the requested URL any time there's a 404 error. I put some analytics JS on that page and track what pages people are hitting. 

Unfortunately, cleanurls rewrites my 404.html with a meta refresh file and redirects the user to the new location at /404/. Because it was a meta refresh, the original URL is gone from both `window.location` and `document.referer`

So this adds support for  a `cleanurl: false` metadata that instructs the cleanurls plugin to not rewrite whatever document it's on.

I also updated the unit tests with a `dirty.html` with this metadata and I updated the contributing guide and made docpad a devDepenency.