Better associationset loading - with tests

I needed options on `AssociationSet#load`, for which there is a pull request #640 by @adamjacobbecker. He didn't have time to finish it, so I took the liberty to continue his efforts.

A little reminder:
>This allows loading associated models with options, like: user.get('comments').load {body: 'hi'}, (err, comments) ->.
>
>It also passes the env to the callback, so you can access other response parameters.

I've added and fixed the tests and merged with the current master. @kristianpd @hornairs @nickjs is this okay now, or do you want more?