Added support for :only and :except methods.

It is possible now to filter returned attributes and associations by:

  UserSerializer.new(user, only: [:first_name, :last_name])
  UserSerializer.new(user, except: :first_name)
allow singular embed when serializing association

Having

```ruby
has_one :post, embed: :ids
```

looks especially weird as it's a one-to-one association, hence there is
only one id. The following looks better:

```ruby
has_one :post, embed: :id
```
add support of mongoid collection serialization
Merge pull request #294 from goshakkk/has-one

allow singular embed when serializing association
Merge pull request #293 from goshakkk/mongoid-support

add support of mongoid collection serialization
Merge pull request #283 from morgoth/add-only-and-except-options-support

Add only and except options support