Add discounts class

Issue #53
Fix python 2.6 error
Add simple discount model

Issue #53
Register discounts in admin

Issue #53
Add discounts into views

Issue #53
Display discounts in templates

Issue #54
We need some template tag for displaying all modifiers in djanog-prices.
Simpler discounts with proper logic flow
Apply just the highest discount
Only try to find the maximum if there are values
Don't allow discounts higher than the product price
Display undiscounted price if different
Merge pull request #56 from mirumee/features/discounts

Add simple discounts