#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 don't like the "configuredBy" as part of the annotation name. Its a very good parameter name (and in line with our "qualifiedBy").  Perhaps: `@InverseMappings` or `@ReciprocalMappings` (math).. 

So: 
```java
@ReciprocalMappings(configuredBy="foobar")
@Mappig(source="paramX", target="paramY")
```
Anyway.. The idea is that you include a set of mappings.