Rebuilt and fixed FieldValueTransformer

The introduction of TypeTransformers lead to the removal of features that were available in release 1.0 and introduced several bugs:

- transforming a String to a collection of any type of objects was not possible anymore
- BUG: no deep matching of target and source type generics, so e.g. a collection of Strings would have been written into a Collection of different Objects without verification
- BUG: exception when getting no (i.e. null) value from FieldValueExtractor, which however is perfectly legit
- primitives could not be converted to other primitives, addition of respective transformers would have lead to lots of different combinations
- user suggested transformers would not be applied if source and target type already matched (-> maybe introduce content transformers)

Those should be (more or less elegantly) fixed now.