XmlHelper: suppress adding namespace

* add readElement() and writeElement().
replace keySerializer, valueSerializer

* SerializableDictionary
* replace keySerializer.Deserialize() with XmlHelper<TKey>.readElement()
* replace valueSerializer.Deserialize() with
XmlHelper<TValue>.readElement()
* replace keySerializer.Serialize() with XmlHelper<TKey>.writeElement()
* replace valueSerializer.Serialize() with
XmlHelper<TValue>.writeElement()