DynamicProperties to be serializable.

DynamicProperties are in general properties of NodeEntities or -relationships, which are supposed to be serializable(?!). Using DynamicProperties makes a NodeElement having a field which itself is not serializable. This patch fixes this issue.

I have recognized this suggestion for improvement because of findbugs integration in my project (which uses spring-data-neo4j) ([ef] is of type DynamicProperties):

  Class [ab.cd] defines non-transient non-serializable instance field [ef]

  This Serializable class defines a non-primitive instance field which is neither transient, Serializable, or java.lang.Object, and does not appear to implement the Externalizable interface or the readObject() and writeObject() methods.  Objects of this class will not be deserialized correctly if a non-Serializable object is stored in this field.