Add session management

[gunthercox]

• Create new session objects
• Allow integration with Django sessions
• Add tests for sessions
• Update documentation

Closes #372

[vkosuri]

@gunthercox will it consider other bot session objects, for example gitter/microsoft session object was expired then chatterbot session object will expire?, will this case is considered?

[gunthercox]

If two bots are created, each will have sessions separate from each other.

bot1 = ChatBot()
bot2 = ChatBot()

For each bot, the sessions could be shared across multiple adapters as long as there is a shared session id. Right now this is something that the developer would have to handle manually because a ChatBot instance has no way of knowing that a Gitter user is the same person as a user on the Microsoft Bot Framework. To handle this, the developer would likely have to store associations between a unique identifier and a user's verified usernames for each service (Gitter, HipChat, Microsoft Bot Framework, etc.).

Hopefully this answers your question. I'm planning to add more documentation on this to hopefully make it clearer.

[vkosuri]

Nice, Thank you