Add Multi-user support

[rmdort]

Multi-user support in the core would be really useful

[gunthercox]

I believe there is actually two cases here, both of which I'd like to implement.

1. One bot is having many different conversations with single users (https://github.com/gunthercox/ChatterBot/issues/372).
2. The bot has a conversation with a group of users. The users may address the bot or each other. (This is a bit more complex than the first case).

[rmdort]

Currently how i am handling user-bot conversations is

1. When a user is connected, i create a user_id cookie and save the user to mongodb
2. I record all the user's conversations in mongodb tagged to the unique user_id, so next time i can pick up the conversation where it left off (TODO)
3. Users current_intent and previous_intent is stored as well. So that the bot knows the context before responding to the user. Previous intent/Current intent is only stored for 5 minutes. (TODO)

group of user < -> bot sounds more complicated