Add xstream serializer

[isears]

I recently used this project to generate xstream payloads for a POC I wrote. I added an extra command line option to allow users to specify xstream serialization, and a corresponding function in the Serializer class to serialize to XML rather than builtin Java serialization. Was hoping there would be interest in incorporating these changes in the main repository.

[frohoff]

This is planned as part of #38 which also requires a framework for pluggable serialization formats as well as testing/validating which chains work with which formats.