Specifying website root for generated docs to fix broken links

Attempts to fix broken links to API docs and tutorial (e.g. links [here](http://frank-fs.github.io/reference/index.html) from [here](http://frank-fs.github.io/frank/)), and broken image ref on api doc's [index.html](http://frank-fs.github.io/frank/reference/index.html).

Not sure if this change will work, but thought I should at least attempt a fix at the same time as raising the issue. :)