Merge lastest

1. Added some missing parts of method populateSentence. Fully tested it and all helper methods of it. Encapsulated all helper methods of method populateSents into a utility class PopulateSentsUtility.

2. Encapsulate all internal data structures represent the tables in MySQL into one DataHolder class.

3. Method characterHeuristics, along with a number of helper methods of it have been added/tested. Fixed a few bugs. New test cases were added to each of them and they are now tested thoroughly.

4. Implement two methods defined in ITerminologyLearner interface. Test cases of the have been added and tested in the meantime.

5. Issues 15 & 21 done.