Allow specifying default id column of models
Use static imports of models in TestData to improve tests readability
Changed the syntax for defining the one-to-one relationships
Allow defining id column per relationship for one-to-one and recursive relationships

I hit some weird bug related to non-static inner classes and generics, and as a workaround I've converted the entire builder API to static classes. The code is truly abominable and should be cleaned up and extracted to another package.