Task: move training code into a main function

In the future, other modules may want to train a classifier. Therefore,
having the code in a main function allows the function to be called
rather than forcing the module to be run from the command-line.
Task: rename main.py to train.py

To prepare for additional sentiment analysis tasks or features, such as
loading a pre-trained classifier and hosting a web interface to query
the system, we are renaming this file to indicate that it is used for
training.