Issue #49
Classes for holding statistics of tags implemented.
Issue #49
1) Copy constructor for TagStatisticsCollection implemented.
2) `get' method of TagStatisticsCollectionIterator no longer reutrn pointer.
   It was a wrong thing to do.
Issue #49
Method `get_tags_statistics' implemeted in Database layer.
Issue #49
`get_tag_statistics' added to the Engine and Front End layers.
Unit Tests added.