Support array methods on embedded document

```
var ChildSchema = new Schema({
});

var ParentSchema = new Schema({
children : [ChildSchema]
});
```

Besides of defining methods on each `child` document by `ChildSchema.statics.doSomething` and `ChildSchema.methods.getSomething`, I want to define methods directly in the `parent.children` array, something like:

```
ChildSchema.arrayMethods.mySortFunction = function () { ... };
ChildSchema.arrayMethods.getEldest = function () { ... };
```

It could be a great feature

The issue names ChildSchema.arrayMethods and the parent.children embedded-document array, but no files or tests. Start by reviewing Mongoose's embedded-document array API and existing array-method behavior, then clarify whether methods such as mySortFunction and getEldest should be inherited and how they should behave. Done means the supported API is defined, implemented, and covered by tests.

Written by the indexing model from the issue text.