#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.
Yes, I was also thinking about how to resolve the ambiguity issue in case two forward methods exist with the same parameter/return type.

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. Otherwise, an error would be raised at the affected reverse method. If there is no ambiguity (which should be the most common case), only `@ReverseMappingMethod` (i.e. without the property) would have to be given.