Allow for nesting of singleton belong_to's.

Note that
    class ThingAController < InheritedResource::Base
        belongs_to :thing_b, :singleton => true
    end
used to mean there was only one ThingA (so resource_path == /thing_bs/123/thing_a), while it now means there is only one ThingB (so resource_path == /thing_b/thing_as/123)

To specify there is only one ThingA, you'll need
    defaults :singleton => true
update pr #194 to merge cleanly