Add loopback.urlNotFound() middleware.

The middleware should be used as the last 3-parameter middleware (regular
request handles) before any 4-parameter middleware (error handlers).

This way a request to an URL not handled by any middleware is converted to
a 404 error that can be handled by whatever error handling strategy is
configured in the application.

See senchalabs/connect#954 for more details.
Merge pull request #70 from strongloop/feature/return-standard-error-for-unkown-url

Add loopback.urlNotFound() middleware.