(pouchdb/pouchdb-server#72) - Reintroduce app.couchConfig

Alright, so this reverts some over eager refactoring that we're not going to need now that express-pouchdb will likely stay one package (although that package will provide a way to select which parts of it is enabled, see #122). Doing so, app.couchConfig is reintroduced which is a first step to fixing pouchdb/pouchdb-server#72.

Based on #143 so tests could be added for express-pouchdb's API (which CouchDB doesn't share, of course).