Added all test cases to method getSentenceHead(). Fixed two bugs in the method.
1) Added method getSentenceHead() to method getFirstNWords(). 2) Fixed a bug in method getPOSptn().
Added several test cases of method getSentenceHead(). Fixed several bugs of it.
updated readme
1) Renamed method addClusterstrings() to addClusterStrings(). 2) Added new content to readme file.
Added Apache Commons Lang library into the project.