Add docpad's layouts path to the swig root paths

This fork adds docpad's layouts paths (`src/layouts/`) to the swig `root` config in addition to the current file's path.

This way it becomes possible to have a swig file like:

    src/layouts/macros/main.html.swig

and reference it in a document file such as:

    src/documents/subdir/somefile.html.swig

like this:

    {% import 'macros/main.html.swig' as macros %}