Feat ordering by associated models now works

It's now possible to order by associated models. This technically might
have worked before b/c we were parsing JSON sent to `order`, but I'm
pretty sure it wouldn't actually work b/c we never grabbed the actual
model to associate by. Regardless, this actually enables things and adds
tests to prove it.

Note: there's a sequelize bug that's poorly reported but definitely
known where `order` with associated models can fail because the sql
generated doesn't include a join. So, I added docs noting that and a
`test.failing` so that we'll be notified when that bug is fixed and can
remove the note.

README.md

@@ -172,6 +172,22 @@ Team.findAll({order: [['name', 'DESC']]})
 Team.findAll({order: [['name'], ['city']]})
 ```
 
+You can even order by associated models. Though there is a [sequelize bug](https://github.com/sequelize/sequelize/issues?utf8=%E2%9C%93&q=is%3Aissue%20is%3Aopen%20order%20join%20) that might prevent this from working properly. A workaround is to `&include` the model you're ordering by.
+```js
+// returns the players ordered by the team name
+// GET /players?order[0]={"model": "Team"}&order[0]=name
+
+// results in a Sequelize query:
+Player.findAll({order: [[{model: Team}, 'name']]})
+
+// if the above returns a Sequelize error: `No such column Team.name`,
+// you can work around this by forcing the join into the query:
+// GET /players?order[0]={"model": "Team"}&order[0]=name&include=team
+
+// results in a Sequelize query:
+Player.findAll({order: [[{model: Team}, 'name']], include: [Team]})
+```
+
 
 ## Authorization and other hooks
 You can use Hapi's [`ext` option](http://hapijs.com/api#route-options) to interact with the request both before and after this module does. This is useful if you want to enforce authorization, or modify the request before or after this module does. Hapi [has a full list of hooks](http://hapijs.com/api#request-lifecycle) you can use.