implement serialVersionUID for all classes implementing serialize interface

[FlorianZipser]

"However, it is strongly recommended that all serializable classes explicitly declare serialVersionUID values, since the default serialVersionUID computation is highly sensitive to class details that may vary depending on compiler implementations, and can thus result in unexpected InvalidClassExceptions during deserialization. "

see http://docs.oracle.com/javase/7/docs/api/java/io/Serializable.html