User: use User.http.path

Use the new property introduced by a recent change in
loopback-datasource-juggler instead of building the URL
manually from pluralModelName.
Register exported models using singular names

Remove the inconsistency between model names used by LoopBack app and
datasource-juggler (modelName, e.g. User) and the name used by
strong-remoting (pluralModelName, e.g. Users).

This way the class name in the strong-remoting metadata can be used
by client-code generators. Before this change, the generators would
produce method names like `Users.login`.