Nested paths for methods in schemas

I'd really like to be able to provide nested paths for methods on a schema the same way it is possible to do with virtuals.

A very contrived example of this would be:

```
var person = new mongoose.Schema({
name: String,
photo: { src: String, width: Number, height: Number }
});

person.method('photo.tag', function(alt) {
return '';
});
```

... while in the above example you could use `photo_tag` as the name of the method, my schemas are a lot more complex than this, and some methods are logically associated with certain nested paths.

Having had a look at how methods are implemented - setting `model.prototype[path]` in `Model.compile` - it seems it would be possible to add support for nested method paths, and I'd be happy to have a go and submit a pull request.

I do want to make sure this wasn't a specific design decision first (or see if you know of some reason it couldn't be done).

Cheers,
Jed

Start at Model.compile, where schema methods are assigned to model.prototype, and compare its path handling with the virtuals implementation. The issue names no file or test path, so trace those entry points first; done means the nested photo.tag method in the example is available and behaves as requested.

Written by the indexing model from the issue text.