2 2 dev dollar off line item promotions

Original Pull Request to Spree: https://github.com/spree/spree/pull/5007

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This allows store administrators to create dollar value off promotions, like $5 off all Ruby on Rails merchandise.  The flat rate calculator for line items will currently allow an admin to deduct a flat rate off of the total regardless of how many items have been ordered.

For example, a FlatRate of $5 off mugs results in the following on an order:
![screenshot from 2014-07-17 14 53 24](https://cloud.githubusercontent.com/assets/764390/3620205/01368494-0dfd-11e4-941e-fd15e95f2cf5.png)

It gives $5 off per line item matching the rules of the order, regardless of quantity ordered. If the desire is to give $5 per order, the Flexi Rate calculator does a great job:

![screenshot from 2014-07-17 14 54 14](https://cloud.githubusercontent.com/assets/764390/3620207/0137694a-0dfd-11e4-8346-ec79111a6072.png)
![screenshot from 2014-07-17 14 54 26](https://cloud.githubusercontent.com/assets/764390/3620206/0136c760-0dfd-11e4-87f5-7b27fd947e58.png)