Stackato manifest

With this commits, we can easily host scanjs on a stackato instance (which Mozilla, incidentically has).
An important change is that server.js would only host the `client/` if you want the URL to be half-way decent i.e., https://scanjs.paas.allizom.org/

This also fixes #24. An additional task would be to point our README to this other URL and forward the github pages. The new hosting environment has https :lock: ;))

What do you think? :)