NEMO-2630.  Add validations to decimals
using constant in models for database limits, underscore to make easier to read
extracted validation logic to module
Merge branch '2-2-nemo-stable' of github.com:godaddy/spree into new-2630
promo_total must be negative instead of positive, tests passing
Merge pull request #21 from godaddy/new-2630

NEMO-2630: Add model validations for ALL decimal values in Spree