Adding ctx.state.user support

[gingerich]

Addresses #57

• Expose the authenticated user via ctx.state.user.
• Add deprecation warning when accessing ctx.req.user
• Updated unit tests

This conforms to Koa v2 best practices for passing data through the middleware stack, https://github.com/koajs/koa/blob/v2.x/docs/api/context.md#ctxstate

Note: npm install to grab new dependency

[rkusa]

Well done, thank you very much!

[rkusa]

FYI: I've updated koa-passport to save the user only in one single place, which is ctx.state.user. Passport strategies are still able to access it through req.user, but users aren't. Since, because of other changes, I am planing to bump the major version (to 3.0.0) anyway, I think I'll not add a deprecation warning. But this is open for discussion.