Expose SqlObject dispatch machinery for public use

[qualidafial]

I propose making the following SqlObject types public:

• Handler
• @SqlMethodAnnotation and HandlerFactory
• @SqlMethodDecoratingAnnotation and HandlerDecorator

This will allow users and library authors to create their own SQL method handlers and decorators, which will in turn make caching feasible as a bolt-on option for users who want it, although there may be other use cases we haven't thought of.

We should also introduce a way of specifying order for when there are multiple decorators. My gut tells me that a static @Priority annotation is not the best approach, since users might want different ordering in different scenarios.

[qualidafial]

For multiple decorators, one approach might be:

@interface DecoratorOrder {
  Class<? extends Annotation>[] value();
}

Thus, in a sql object:

@FooDecorator
@BarDecorator
@DecoratorOrder(BarDecorator.class, FooDecorator.class)
List<Thing> listAllTheThings();

If you don't specify an order, then decorator order is undefined.

[stevenschlansker]

OK by me.

[qualidafial]

Fixed in #458