Clean up training code

[gunthercox]

This makes the following changes to the code in the training module:

• Variables have been renamed to be more accurate and descriptive.
• The ChatterBotCorpus trainer is no longer dependent on the ListTrainer.
• The entire statement history for a conversation is no longer stored (only the last statement is ever needed).

[gunthercox]

Should have been a equal sign 🥇

[vkosuri]

@gunthercox Master, How about list comprehension, iterating each list will take long time if we train with huge data

[gunthercox]

List comprehension is always an option to improve performance. I do have concerns about readability though as the format of list comprehension is often difficult to understand at a quick glance.