1. Change abstract class "ModelBase" to interface "Model" and remove
loadTrainData(), loadPredictData, savePrediction().
2. Fix CompareTo() bug in RankPriorityQueue.
3. Modify Tagger.Main() in response to ModelBase change.
4. Modify Cooccurrence in response to ModelBase change.
5. Remove StackExchangeData. This is useless after the change of
ModelBase.