Update and rename task3.py to task3b.py

#
# 1. The solution is similar to the method for sentiment analysis, see below  
# 2. show how to split sentences and generate word features
# 3. raw data is transformed into a new training data, different from the method used in textblob;
# ref: http://www.laurentluce.com/posts/twitter-sentiment-analysis-using-python-and-nltk/
#
# note: however, the performance is poorer than that by textblob, from empirical results