Add on_rollback hook to resources

[wadetandy]

Replaces #26. No material change at the moment.

From @jsonapi-suite/jsonapi_compliable#137, this allows an API
maintainer to run a rollback hook on a given resource if an error occurs
after the resource's before_commit hook is successful, but before
the transaction actually completes. This will be helpful in situations
where a user would like to rollback something that cannot be
automatically undone in the transaction. The on_rollback hook will
receive as its argument the return value of its before_callback hook
(or the created/updated/deleted record if no before_callback hook is
provided).

class EmployeeResource
  has_many :positions

  before_commit only: :create do |record|
    # Makes HTTP request and returns ID of created record
    PayrollSystem::Employee.create(record)
  end

  on_rollback only: :create do |sap_record_id|
    # Delete created record
    PayrollSystem::Employee.find(sap_record_id).destroy
  end
end

class PositionResource
  before_commit do |record|
    raise 'boom'
  end
end

If creating an employee and a nested postion, the position will blow up,
and the rollback hook will clean up after itself.

[richmolj]

@wadetandy let's move this to the main Graphiti repo when you get a chance. No rush, but I like the work here.