Add support for third party session handlers

[kirsle]

This adds a constructor option sessionHandler which provides a hook for third party developers to implement their own session stores (e.g. using Redis or something).

The default session handler, from src/sessions.coffee implements the in-memory store that RiveScript previously used. All internal code references to user variables from RiveScript._users have been modified to go through the public setUservar() function and friends. These functions then delegate to the equivalent functions from the session handler. The core RiveScript library provides no other session handlers, but some may be added as examples in the eg/ directory in the future.

Session handlers aren't required to implement all functions (the unit test tries setting a "null" session handler which gives the bot really bad amnesia).

Fixes #135

[gleuch]

👍 Great!

[julien-c]

👍 Nice. Do you know whether you will merge and release this soon?

[kirsle]

I'm not sure how useful this was after all. It won't work with any async session drivers, which sort of defeats the original purpose for this code. Somebody wanted to hook up a database layer (Mongo or Redis or SQL or something) to manage the sessions, but most of those things work asynchronously using promises or callbacks, but that can't work with the way rivescript-js is currently designed.

[julien-c]

Got it. I'm in the process of implementing the way you're doing it in the persistence example, looks like it's going to be fine. Thanks!