#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.
I agree on what you say about the hierarchy and elements taking precedence, but I'm a bit concerned about the name `@MappingMethod`.

The problem I see is that each method of a mapper interface is a "mapping method", whereas only some of them (reverse ones) would be annotated with that annotation. At what other places see you potential for re-use of the annotation if it where named more generically? `@ReverseMappingMethod` seems less confusing to me, unless there is another compelling case where `@MappingMethod` would be (re-)used.