Allow a custom URL for associations

[swilliams]

(Inspired by #34)

Right now:

class Hat
  include Arkenstone::Document
  url 'http://example.com/hats'

  has_many :feathers
end

class Feather
end

Will fetch a hat's feathers by calling http://example.com/hats/:id/feathers. Add an option to the association declarations (has_many, belongs_to, etc) to override the associated model name:

class Hat
  include Arkenstone::Document
  url 'http://example.com/hats'

  has_many :feathers, entity_name: 'turtles'
end

Which'll use build this url instead: http://example.com/hats/:id/turtles.

[m3talsmith]

I would like this actually. Also, this could help the has_many :through idea in #14

[m3talsmith]

Is this still a need?