REST call of DataAccessObject.findById returns 404

Modify the remoting configuration of `DataAccessObject.findById()`
and add a `rest.before` handler that converts `null` to 404 error.

The change is covered by a test in loopback project.
Merge pull request #44 from strongloop/feature/get-unknown-id-should-return-404

REST call of DataAccessObject.findById returns 404