SerializableObservable

Originally, serveral classes exteded the java.util.Observable class.
However, this interfered with redo/undo fucntionality which has been
implemented by serializing/deserializing. As a result, the
SerializbleObservable class was created. It extends the Observable class
but also implements the Serializable interface.
Merge branch 'master' of github.com:tmzhuang/pvz-reloaded