Remove read_only parameter at database level

[gunthercox]

The ChatBot class currently takes a read_only parameter that controls if the storage adapter can write to the database. This needs to be changed so that the read_only parameter only controls statements being saved at the ChatBot class level (essentially read only just controls if the chat bot can learn).

The reason this change is needed is because:

1. It is largely inconsistent with how Django handles it's storage and as ChatterBot improves support for integration with Django supporting a passive read-only database option will become increasingly difficult.
2. Changes have been made in the past to allow training to occur when read_only is set to True. (The addition of a force parameter to the storage adapter). Removing storage adapter-level write access control will allow this code to be simplified.

---

This also simplifies storage adapters and makes it easier for developers to create new ones.