Add better integration methods to App

[craigbeck]

Adding derby-webpack to the ecosystem highlighted some rough edges in setup with the existing API.

1. writeScripts needed to be called, even if no scripts were boing written by derby, just to install listeners for app refresh
2. backend.modelMiddleware() needed to be added to the express app, or derby would default to creating a new model that lacked a connection, so was broken (will look into this separately)

Solutions

1. Add app.initAutoRefresh(backend) to setup listeners (better naming, not calling obtuse or private method)
2. Add app.middleware(backend) method as obvious express integration point that sets up both the model and the router. If other middlewares is needed, existing methods backend.modelMiddleware() and app.router() are still available to add individually.