Convenient managment of conflicting choices
Extend admin interface for more convienient user group management
Support specifying number of items in the package

Introduces new field in CSV - "items_per_pack"
This feature may be used in two ways:

1) To sell items in packs. For example Business Cards may be sold in 10, 20,
50, 100, 300 ... etc quantities. If you specify
quantity=300 and items_per_pack=300 in CSV line - the system will treat
that line as "price for 300 business cards=X*1". But, since we calculating
"packs" - if quantity requested by user is 301 - the price will be calculated
for 600 cards (X*2 two packs needed). It won't make any problems for
discrete pricing - because user can only select from a list of predefined quantities

If you use quantity=300 and items_per_pack=1 - the system will try to calculate unit price first,
like "price for 300 business cards=round_to_two_decimals(X/300)*300"
introducing some rounding errors, but quantity may be specified with
one unit precision like
"price for 301 business cards=round_to_two_decimals(X/300)*301"

2) To sell items by sheets. For example Stickers - one sheet may contain 50 stickers. So we need to specify quantity=1 and items_per_pack=50, so
when user requests 60 stickers price calculated
like "price for 60 stickers = ceil(60/50)*X" where X is price for one sheet.

For ordinary products use quantity=1 and items_per_pack=1 in CSV price line

In CSV quantity has to be either 1 or (bigger or equal to items_per_pack)

items_per_pack > quantity > 1 in CSV does not make sense and should not be used
More convenient Price option_choices management
Fix basket price calculation for packs
Minimal price support
Merge branch 'master' into misc_changes
Proper price and unit number calculation
Correct display of quantity
terminology change