Test reading/writing different versions of objects

Some tests need to be added to be able to reply to http://infinispan.blogspot.com/2011/04/first-50-beta-now-available.html?showComment=1302265406191#c8477745491388109221 in a more specific way.

A test needs to be added for a new version of an object that adds a new field, and see if it can be read/written correctly under different circumstances: extending Serializable, extending Externalizable and via our internal Externalizer implementation.

Any findings need to be documented.