#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.
Hum, looking at the example mappers I'm starting to get doubts about `@ReverseMappingMethod`. Specifically, I don't like the notion of "reverse" (and thus implied "forward") mappings. Which direction is which really seems open to interpretation.

@sjaakd, WDYT about `@ConfiguredByInverseMethod` / `@ConfiguredByInverseMethod(name="foobar")`? "Inverse" does not put emphasis on which direction of mapping this is.