Response list structure

[gunthercox]

Each statement object that ChatterBot stores contains an attribute for referencing each statement that it has been used to respond to.

In the future, the in_response_to list should become a dictionary of the response statements with a value of the number of times each statement has occurred. This should make selecting likely responses more accurate because we can see how common one response is to another, instead of just the number of times that response has occurred.