#207 adder method for collections on target

For review, it is not yet completely finished. It contains TODO's in the `MapperCreationProcessor.java` and `Type.java`. It also lacks more unit test (placeholders are in place).
Ok. good point. I always assumed there would be a setter method (or, if not, an initialized getter). I never considered there would 'only' be an adder. 

Do you still want to be the 'getter' method the final fall back if there's no suitable setter (and we're not allowed to use the adder)?

I assume the `SETTER_ONLY` is still the default option? Because we have a common config I think we need to do it like this:

```java
enum CollectionMappingStrategy {
    DEFAULT,   // SETTER_ONLY
    SETTER_ONLY,
    SETTER_PREFERRED, // use setter if setter and adder are present
    ADDER_PREFERRED;  // use adder if setter and adder are present
}
```
Only then you can distinguish the situation where somebody explicitly want to override the setting in in the common config (applied to a set of mapper) by the specific config in a mapper, WDYT?