Return ActiveRecord::Relation instead of Array

Also includes some spec cleanup
Restore statement not null condition
Use #pluck instead of #map
Bring PurchaseOrderAccount in line with CreditCardAccount
Bring purchase_orders spec in line with CC
Move common specs into shared_examples
Extract common code into OrderDetail
Begin phasing out @authable
Extract need_reconciling into a concern
Merge pull request #148 from tablexi/selected-account-updates-ar

Have CreditCardAccount.need_reconciling return an ActiveRecord::Relation instead of an Array