[Feature] Policy Serializer 

Taken from the updated **README.md**:

## Serialized Policies

In some applications you might want to provide a client API. Pundit helps
provide your authorization policies as first class citizens, by giving you
a way to convert a policy to a hash. It's easy, just include the serializer
module and you're good to go.

```ruby
class MessagePolicy

  include Pundit::Serializer

  attr_reader :user, :message

  def initialize(user, message)
    @user = user
    @message = message
  end

  def create?
    not message.new_record?
  end

  def destroy?
    message.user == user
  end

end
```

Now you can call `#to_h` on the policy with the module included.

```ruby
@message_policy = MessagePolicy.new(current_user,@message)
@message_policy.to_h
# => { create: true, destroy: true }
```

**Note:** For the serializer to work, you need to use the pundit naming
convention for queries with the interrogation point in the end `index?`

The serializer also works for inherited policies.