scope-like remotable metadata for belongsTo

Modify the methods created by `belongsTo` relation to mimick the methods
created by scopes (e.g. via `hasMany` relation).

This allows client code generators like loopback-angular to use the same
code for all relation types.
Merge pull request #99 from strongloop/feature/scope-like-rest-methods-for-belongsTo

scope-like remotable metadata for belongsTo