Thunk support

This pull-request makes the mquery an inherit from a function (thunk) which will trigger an `instance.exec`. This is a bit of a hack, so I completely understand if you don't want to merge it! That being said, it makes mquery work well with [co](http://github.com/visionmedia/co) right out of the box, which coupled with the fluent API makes for awesomeness. It allows you to do the following:


```js
var users = yield mquery(userCollection).find({name: 'Steve'})

//or

var updateCount = yield mquery(userCollection).where({name: 'Bob'}).update({name: 'Steve'})
```

It does still maintain support for the callback syntax.

Thanks for the awesome library either way.