#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.
Do you mean a container annotation? What would be the use case for
specifying @ReverseMappingMethod several times? In my picture it applies to
a method, not a single property. Or what have you in Kind?