Mage 6404: Products list sorting by a custom attribute works incorrect

Issue #58 turned out to be a real treasure because of dependencies between Mage_Eav and Mage_Catalog ;)

Attributes that use the table source model actually worked, but the boolean source model and the countryofmanufacturer source model where not sortable.

This branch adds the required methods required to make attributes with the boolean and the countryofmanufacturer source model sortable, and also make them work with collections on the frontend with the flat catalog enabled.

Integration tests are implemented with and without the flat catalog enabled, for all three source model types:
* eav/entity_attribute_source_table
* eav/entity_attribute_source_boolean
* catalog/product_attribute_source_countryofmanufactuer

All changes should be 100% backward compatible.