Allow model-based associations for `belongs_to` and `has_one`

[dmcinnes]

Does it make sense to have model-based association on belongs_to and has_one like they are available on has_many

class API::V1::Post < Daylight::API
  has_one :blog, through: :association
  has_one :company, through: association
  has_many :comments, through: association # only one currently supported
end

[dmcinnes]

Copied from https://github.com/att-cloud/daylight/issues/7