#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 just started implementing..  One more remark though... We have `@Mappings` as forward case.. Shouldn't it be `@ReverseMappings` then?