Refactored has_many to add :class option

Example

  class RemoteResource < LogicalModel
    has_many :child_resources, class: 'TheClass'
  end

  RemoteResource#child_resource returns array of TheClass

DEPRECATION!!!

  has_many_keys = () should not be used

  class RemoteResource < LogicalModel
    self.has_many_keys = [...]
  end
Version bump to 0.5.11
Regenerate gemspec for version 0.5.11