[feature] Address rule with subtotal inclusive tax

Extends the shopping cart address rule with an attribute for 
subtotal inclusive tax.
[task] Improve README.md
[bugfix] Processing customer groups of gift rules

Fixes a typo, which prevents that gift rules for customer groups
beside 'NOT LOGGED IN' have no effect.

related #1
[task] Add modman file

related #4
[bugfix] Out of stock gifts

If your gift is out of stock, you are not able to fill your cart,
when the rule matches. So we need to just skip adding the gift.

related #2
[bugfix] Empty gift labels

Avoid adding messages if there is no gift label at all.

related #3
[task] Increment version number
[task] Merge development branch

 * Added address rule condition for subtotal inclusive tax
 * Fixed processing customer groups of gift rules
 * Fixed adding empty gift labels
 * Fixed adding out of stock gifts