#252 merging reverse mapping with forward mapping.

There is also an error in the `Mapper#reverse` method class.  The check on constant / expression has been fixed.

Also, the order of forward / reverse method is not relevant (as thought before). This is been assured by adding some tests.
> Yes, I would apply all the mappings from the corresponding forward mapping
> method.
So: is `@ReverseMappings` not a better name then. You could consider that annotation as a placeholder for all forward mappings. 

> How would you select the right forward method without the property?

Well: we are doing it now as well, right?. The big difference with the current situation is that we do not select a method automatically anymore, but only if it carries the annotation.