#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.
> With that, you mean also the current implementation? I agree. I think we should make it explicit. Suggestions?

Yes, I mean the current implementation. WDYT about requiring the forwarding method to be explicitly referenced like so:

    OrderDto orderToDto(Order order);

    @ReverseMappingMethod(configuredBy="orderToDto")
    @Mapping(source="foo", ignore="true)
    Order orderDtoToOrder(OrderDto orderDto);

I don't think we need the qualifiers here, a simple reference "by name" suffices IMO.

The configuration of the reverse method would be merged with the one from the forward method as you say (giving the own configuration precedence over the one from the forward method).

I'd restrict it to the same mapper only, I think having reverse/forward methods on two different mappers seems rather exotic so we can ignore it. Using just the name as reference should be good enough (in conjunction with source and return type) to identify the forward method.

> About this PR. What I'm trying to achieve is that you don't have to redefine mappings you provided earlier, once you decide to do one thing differently in the reverse mapping. 

+1 That makes sense.

If we agree on that approach, do you want to update this PR accordingly?