Improved polymorphism support

• Fix depending on a fully generated builder from a manually maintained builder.
• Pick up fields on implemented interfaces, so you don't have to @override them.
• Add BuiltValue annotation for specifying settings.
• Add "instantiable" setting. When false, no implementation is generated, only a builder interface.
• Polymorphism support: you can now "implement" a non-instantiable Built class. The generated builder will implement its builder, so the types all work.