example: make db connectors an optional dependency

Move database connectors to `optionalDependencies` field of the
scaffolded `package.json`. This way the scaffolded project can be run
against the in-memory db on the machines where build chain (node-gyp)
is not configured correctly.
example: remove models/car.js template

The template was added by a mistake, sls-sample-app does not have
any custom methods for the Car model.
Merge pull request #9 from strongloop/feature/make-connectors-optional

example: make db connectors an optional dependency