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 a 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.

There is a related loopback-workspace pull request coming soon.

/to: @ritch 
/cc: @Schoonology @raymondfeng