[bugfix] Unnecessary overrides in rule resource

Removes unnecessary overrides in rule resource which causes
incompatibility issues between Magento CE 1.7 and 1.8.

related #22
[feature] Minimal support for configurable products

Adds minimal support for configurable products. Includes also
some refactoring of the validator.

related #22
[task] Merge pull request #22

Merges pull request #22 into development but without the minimal
support for configurable products. Includes also some changes
regarding the code formating.

related #22
[task] Merge development branch

 * Provides backward compatibility with Magento CE 1.7
 * Refactors rule validator