Shared partials uses middleman's partial method.

This is related to #27. I have simplified the shared partials code using middleman's existing partial method. However, this requires a renamed of the partial includes in the shared repo – that is, we have to adapt the code in the other repos (happy to do this though).

I'll also issue a PR in the shared repo.

@orta If you see any other problems I hadn't considered with this, please let me know.