Configure models

- Change models.json to configure existing models
- Remove auto-attach

/to @ritch please review
/cc @kraman 

Requires: strongloop/loopback#308
Related to: strongloop/loopback-example-full-stack#13

Before this patch is landed, we have to either back-port loopback 304 & 308 to loopback 1.x (master), or drop loopback 1.x from package.json of loopback-boot.
Not sure what `2.5` is referring to. Are you suggesting we break backwards compatibility at `loopback 2.5` or are you talking about `workpasce 2.5`?