accept nested attributes for a property on an AF::Base object

[jcoyne]

Currently if you have:

class Image < ActiveFedora::Base
  property :creator, predicate: ::RDF::DC.creator, class_name: Creator
  accepts_nested_attributes_for :creator
end

then when you instantiate the object you get:

ArgumentError: No association found for name `creator'. Has it been defined yet?
    from /Users/justin/.rbenv/versions/2.1.5/lib/ruby/gems/2.1.0/bundler/gems/active_fedora-bee58b88a84c/lib/active_fedora/nested_attributes.rb:87:in `block in accepts_nested_attributes_for'
    from /Users/justin/.rbenv/versions/2.1.5/lib/ruby/gems/2.1.0/bundler/gems/active_fedora-bee58b88a84c/lib/active_fedora/nested_attributes.rb:64:in `each'
    from /Users/justin/.rbenv/versions/2.1.5/lib/ruby/gems/2.1.0/bundler/gems/active_fedora-bee58b88a84c/lib/active_fedora/nested_attributes.rb:64:in `accepts_nested_attributes_for'