DB related fixes

After installing node-tileserver on my machine, I ran into two problems. The first was that the code didn't play well with modern PostGIS, which requires the SetSRID function to be prefixed (ST_SetSRID). The second was that the server wanted to connect the DB as user "postgres" which is probably OK for a local deployment, but definitely not good for setting up a public facing server, so I added two config options: "username" and "password" for more flexible Postgres connection. I think that merging these tiny changes will help other users deploying node-tileserver on their machines.