Preferences::Store quacks like a Hash
Refactor preferences, add ScopedStore
Spree::Preferences::Base
switch to serializing preferences on records
remove now unused preference methods

* remove save_pending_preferences
* remove rails_cache_id
Prefereable#default_preferences
s/preference_store/preferences/g
preferences shouldalways include defaults
fix for autosave: true not working with serialized
spec fix
Fix creation of adjustment
Fix some possible race-conditions in backend specs

* adjustments_promotions_spec.rb: wait for first coupon to apply
* properties_spec: allow driver to wait for a second tr
* wait_for_ajax should wait for jquery to be defined
* ensure all requests finish at end of spec