Add response errors to ActiveModel using ActiveModel::Errors

This commit adds the private method `add_errors_to_base` to
`Her::Model::ORM`, which is called in `save` to add any response errors
to the resource's `ActiveModel::Error` collection. This means errors can
be accessed via `errors` on the object and helpers, such as
`full_messages` can be used.

*Example*
```
class User
  include Her::Model
  # validates :name, presence: true (on remote model)
end

@user = User.new
@user.save

@user.errors # => { :name => ["can't be blank"] }
@user.errors.any? # => true
@user.errors.full_messages # => ["Name can't be blank"]
```

- Adds private method to `Her::Model::ORM` to map response errors to an
  `ActiveModel::Error` object
- Adds call to method in `save` to add errors to resource object
- Adds spec to test error mapping
Fixing tests related to af7bc45

- Reformat error hashes in test responses
- Modify `Her::Errors::ResourceInvalid` to use errors object
Add `_ids` has_many association array

This commit adds an `_ids` array getter and setter on `has_many`
associations, which is returned in `to_params`. The association array is
also removed from `to_params`.

**Example**
```
class Post
  include Her::Model
  belongs_to :user

  attributes :title
end

class User
  include Her::Model
  has_many :posts

  attributes :name
end

@user = User.find(1)
@user.post_ids # => [0, 1, 5]
@user.post_ids = [0, 1, 5, 14]
@user.to_params # => { name: "Tobias Funke", post_ids: [0, 1, 5, 14] }
```

- Add `_ids` getter and setter to
  `Her::Model::Associations::HasManyAssociation` on attach
- Add `_ids` array to `to_params` output
- Remove association array from `to_params` output
- Add specs to test presence of `_ids` array and absence of association
  array