Rename `app.json` to `config.json`

The new loopback project layout adds a concept of components like
'rest server' and 'isomorphic client', each component having its own set
of boot files. The name `app.json` is confusing, since it is configuring
a component, not the app (which is the whole project).
Merge pull request #13 from strongloop/rename-app-json-to-config-json

Rename `app.json` to `config.json`