Create fields companion class from all data models.

[serj-lotutovici]

When we move the data module generation to AutoValue #137, we can add a companion class generator, that will allow to build a user fields array. This is useful for resources like UserProfilesResource where the spec accepts model fields that have to be returned by the api.

This will reduce the probability runtime bugs, when using accessing a resource.

Things to keep in mind:

• The api accepts nested field names as well, like field1.sub_field1.sub_sub_field2 the annotation processor will have to lookup for dependent builders.
• The generated builder like class Fields should be something like:

@GenerateFields // Name not final.
abstract class XingUser {
  abstract class Fields {
    Fields field1();
    String[] build();
  }
}

[serj-lotutovici]

cc @MyDogTom