Allow custom relation path (http) - enable hasOne remoting access

Specify `options: { path: 'plural-name' }` to set custom relation paths, regardless of `as`:

``` javascript
// Relation path setting - final api: /users/1/public-photos
User.hasMany('Photo', { 
  as: 'publicPhotos', 
  scope: { where: { public: true } }, 
  options: { path: 'public-photos' }
});
```

See also: https://github.com/strongloop/loopback-datasource-juggler/pull/199