REV 12: Update serialization conventions

This update includes a bump to the Revision, necessitated by 7b5455d04d7acf9faf07e6ecf40f4af25c0e394a, and includes similar and related updates to serialization conventions. These changes are explained below in this new excerpt to BREAKING_CHANGES:

## Revision 12

Several changes have been made to serialization conventions for the
JSON and REST adapters.

### Foreign Key IDs for Arrays

In order to be consistent with singular foreign keys, the REST serializer
now serializes arrays of foreign keys with the singular form of the key name
suffixed with `_ids`. Therefore, just as `author_id` represents a single
author, `author_ids` (and not `authors`) now represents an array of authors
associated with a parent record.

Custom `key` mappings can be configured to override these defaults as needed.

### Sideload by Type

When loading data, the previous convention was to expect sideloaded data
to be included alongside a parent record based on the name of its relationship.

For instance, given the following model:

```js
App.Contact  = DS.Model.extend({
  name:         DS.attr('string'),
  phoneNumbers: DS.hasMany('App.PhoneNumber'),
  homeAddress:  DS.belongsTo('App.Address')
  workAddress:  DS.belongsTo('App.Address')
});
```

... the following payload would be deserialized properly:

```js
{
  "contact": {
    "id": 1,
    "name": "Dan",
    "phone_numbers": [1, 2],
    "home_address_id": 3
    "work_address_id": 4
  },
  "phoneNumbers": [
    {
      "id": 1,
      "number": "555-1212"
    },
    {
      "id": 2,
      "number": "555-2222"
    }
  ],
  "homeAddress": [
    {
      "id": 3,
      "zip_code": "03086"
    }
  ],
  "workAddress": [
    {
      "id": 4,
      "zip_code": "94107"
    }
  ]
}
```

Now, `homeAddress` and `workAddress` will be expected to be sideloaded
together as `addresses` because they are the same type. Furthermore, the
default root naming conventions (underscore and lowercase) will now also
be applied to sideloaded root names.

The new, more consistent and concise conventions for sideloading are:

```js
{
  "contact": {
    "id": 1,
    "name": "Dan",
    "phone_number_ids": [1, 2],
    "home_address_id": 3
    "work_address_id": 4
  },
  "phone_numbers": [
    {
      "id": 1,
      "number": "555-1212"
    },
    {
      "id": 2,
      "number": "555-2222"
    }
  ],
  "addresses": [
    {
      "id": 3,
      "zip_code": "03086"
    },
    {
      "id": 4,
      "zip_code": "94107"
    }
  ]
}
```

Custom `sideloadAs` and `key` mappings can still be configured to override
these defaults as required.