Add store credits seeds.

Without the seeds, things don't work correctly. If you run the
migrations this data will be added, but if a new developer/environment
is created from seeds then things don't work quite right.
Store originator on store credit events

This allows admins, gift cards, reimbursements, refunds, etc to be recorded as the "originator" of a store credit event.

- Add originator_id and originator_type fields on spree_store_credit_events table
- Allow sending an "originator" property via payment method authorize/capture/purchase/void/credit calls
- Add an action_originator attr_accessor on StoreCredit that propagates to the StoreCreditEvent on save
- Send an originator to StoreCredit when redeeming a gift card
- Send an originator to StoreCredit when granting a store credit via the Admin UI

Also:

- Rename StoreCredit#authorization_code -> #action_authorization_code to match the other "action" properties
- Refactor some method calls a bit
- Refactor some specs a bit
Add some specs

Specs I forgot to put in db4a81e7bcd34fee80b2ec6a9df638b780cc0462
Also, update a non-bang save to a bang version. It was failing silently in the specs due to some missing data and was causing these new specs to fail.
Stop using ActiveSupport::Concern

ActiveSupport::Concern does not deal with some changes to Rails 4.1
autoloading, leading to issues with multiple included blocks being
defined when modifying files in the Rails development environment.

Not defining included as a block seems to allieviate this issue, so this
commit strips the use of ActiveSupport::Concern, and just deals with
.include and .prepend directly.

Closes #12.
Revert "Stop using ActiveSupport::Concern"

This reverts commit 7ccf4b684649bce48448ab5efa10f16b2a4254ea.
Stop using ActiveSupport::Concern

ActiveSupport::Concern does not deal with some changes to Rails 4.1
autoloading, leading to issues with multiple included blocks being
defined when modifying files in the Rails development environment.

Not defining included as a block seems to allieviate this issue, so this
commit strips the use of ActiveSupport::Concern, and just deals with
.include and .prepend directly.

Closes #12.
Don't change a gift card's redemption code if it is initialized with one
Add store credit display to checkout flow

- Shows store credit in side/summary bar
- Shows store credit when needing to enter in payment details, while
making it clear that the customer will not be charged
- Shows store credit in the order confirmation page as well as the order
show page
- A customer's order index page still shows the usual order total
(unchanged behavior)
Only show store credit on summary if it exists

Adds in using_store_credit? helper on order_decorator to assist in views
determining whether or not to show store credit