#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.
> So what about having an optional configuredBy() attribute in @ReverseMappingMethod? It would have to be given to uniquely identify the method from which to inherit the configuration in case there are several candidates.

I think that's the way to go.

Only one discussion topic remaining: the name. I'll use `@ReverseMappingMethod` as working name. Once you're on the path of thinking of this annotation as a placeholder for the forward mappings, its hard to loose it :smile:. One more try: what do  you think about `@IncludeForwardMappings`?