PA: Added "only" option for polymorphic assosiations

NOT ADDED BY THIS COMMIT:
    Readme fixes

  Changes to be committed:
	modified:   lib/counter_culture/counter.rb
	modified:   lib/counter_culture/fixer.rb
	modified:   lib/counter_culture/helpers/relation_tracer.rb
	modified:   spec/counter_culture_spec.rb
	modified:   spec/fix_counters_spec.rb
	modified:   spec/models/mark.rb
	modified:   spec/schema.rb