Serializer Improvments

[volsa]

PR #967 will change the Serializer API drastically to make it more concise, however there are still some issues and features we'd like to add in the near future as discussed offline with @ghaith

1. Make the serialize() method compatible with SC; the current implementation for example lacks support for the position element (among other things) which in turn yields a null-pointer exception in SC when copy-pasting the generated XML output
2. Refactor the serialize() method to return a model rather than an XML string, and consequentially add a serialize() method to the model to generate XML. Effectively we can then both test the model and parser separately.