[bugfix] Derivation of product conditions

Product conditions were derived wrongly from a class of the 
Mage_SalesRule module.

related #8
[bugfix] Product attribute conditions

Forces the loading the product attribute if this 
was not already done.

related #8
[bugfix] Multiple processing of gift rules

Prevents gift rules are applied repeatedly at run time.

related #8, #9
[bugfix] Success messages on adding gifts

Not in any case, when a gift is added to cart, success 
messages are displayed.

related #10