#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.
> what do you think about @IncludeForwardMappings?

:smile: It's not bad, but `@ReverseMappingMethod` leaves the room open for other future options which are not directly related to linking the forward method. That's why it has my preference.