#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.
Btw. I'm a bit unhappy about the reverse mapping feature in general. I feel the implicit linking may be surprising to users (as in that post on mapstruct-users).

 Maybe a reverse mapping method should explicitly be "linked" to its forward counterpart somehow, e.g. using an annotation `configuredBy()`? That'd also solve the ambiguity problem above, and it would avoid applying configuration from the inverse method to the forward method.