#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).
For the tests you also could use Guava's `ImmutableMap$Builder`. This reads a bit more compact as you don't need that `init()` method, plus it avoids the anonymous class and the warning about the missing serialization id.