Support lowerCamel key format

You can specify that serializers use the lower-camel key format at the config, class or instance level.

```ruby

ActiveModel::Serializer.setup do |config|
  config.key_format = :lower_camel
end

class BlogLowerCamelSerializer < ActiveModel::Serializer
  format_keys :lower_camel
end

BlogSerializer.new(object, key_format: :lower_camel)
```

Relevant to issue #398. Pull request #436 also offered a solution for changing the key format but that approach is now in conflict with head/master.