Issue #1755: Support Attribute Conversion for Persistence

The @Converter annotation and the Attribute Converter interface have two big benefits:

1) They make it possible to persist any as-yet-unsupported java types that do not implement java.io.Serializable. (If they do, it is possible to persist them automatically by turning on the <persist serialization="true" /> option in the objectdb.conf file). 

2) They also make it possible to persist custom objects that are fundamental to your domain in an efficient manner.