#20 Open defaultTransformers map for modification

[Igor Spasic]

Currently, defaultTransformers (in TransformersUtil) are closed for modification. There is no reason for that. Instead, let users be able to register default transformers that are application-wide. Two reasons for this:

1) now i have to add them each time a serializer is created. yes, i can wrap it, but:
2) performance - default map is used as it is - it is not copied etc. If i have to add default serializer manually every time on serializer createion, then it takes some CPU clocks just for populating the map, every time over and over again. Imagine that for big projects and with many custom transformers.

[Igor Spasic]

This will also solve issue with replacing default Object transformer (reported by me)