Hackathon_DiscountForATweet_Model_Condition_Tweet::validate() checks for == instead of contains

[Schrank]

Actually the method calls parent::validateAttribute() and this is mapped for the conditional verb. Is is actually only possible to choose between == and != so it only is true, when you only twitter what is defined in the backend.

We need a change on the code, so we can choose between ==, !=, contains and contains not