Session Issues
I'm trying to use grant with koa, but my boss is pretty insistent that we not use session store for ANYTHING except the auth flows. So I can't just do what you examples do:

``` javascript
app.use(session(app)
```

I'm using the [koa-joi-router](https://github.com/pebble/koa-joi-router) and I tried this:

``` javascript
router.use('/connect', session(app))
```

to limit it to just the grant endpoints but I keep getting an error that I the session is missing or misconfigured. Based on some logs I added to grant in my `node_modules` folder, it looks like the redirect comes back and grant can't find `session.grant` or a provider. I added this to the callback generator around line 119:

``` javascript
    var grant = this.session.grant || {}
    var provider = config.provider(app.config, grant)
    var flow = flows[provider.oauth]
    console.log('callback invoked', this.session, provider);
```

which spits out: `callback invoked {} {}`.

Any suggestions or workarounds?
