#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.
> No.. don't worry. I was not thinking of a container annotation. But, in
essence you drag in all forward mappings right?

Yes, I would apply all the mappings from the corresponding forward mapping
method.

> Furthermore: I'm a bit worried by the parameter configuredBy. Perhaps
@ReverseMappings (or @ReverseMappingMethod) is sufficient. You probably
need to do some checking anyway if return-type matches parameter and vice
versa. And what if it doesn't match? You need to give an error message for
something that you already know.

How would you select the right forward method without the property?