Display KRW use USD for orders

Features:
- Added support to convert order totals to KRW
- Added support for views with display_presentation_* methods (these return Spree::Money objects)
- Updates trigger recalculation of presentation values

Changes:
- Configuration option secondary_currency renamed to presentation_currency and added additional settlement_currency option 
- CurrencyRate now uses the presentation_currency when choosing the conversion currency
- CurrencyRate has to_f method for returning a fractional from the money object.

To do:
- [x] Add ```display_price(product)``` helper
- [x] Add ```variant_price variant``` helper
- [x] Ensure presentation_item_total is updated along with item_total
- [x] Add conversion from 'KRW' to 'USD'
- [x] Add presentation_single_money to the line_item class