Adding TweetReader (which is right now also processsing all the tweets)

1. Adding TweetReader (which is right now also processsing all the
tweets)
> Reads the Tweet line by line
> Select the Tweet Message (NEEDS ENHANCEMENT)
> Preprocess the Tweet Message (Pick important words, NEEDS ENHANCEMENT)
2. Using json-simple library (https://code.google.com/p/json-simple)
3. Added AppConstants to manage application-wide constants at a single
place.