Add methods to ERXKey that simplify creation of ERXExistsQualifiers.

Allows for easy creation of the sometimes-confusing ERXExistsQualifier. With these changes, you will be able to create an EOQualifier that will filter Person objects for ones that have related Pet objects or Person objects for ones that have related Pet objects that have their isDog attribute == true (i.e., People that have a pet dog) using:
EOQualifier thatDoNotHaveAPet = aPerson.PETS.isEmptyRelationship();
or
EOQualifier thatHaveAPetDog = aPerson.PETS.atLeastOneSatisfies(Pet.IS_DOG.isTrue()); 
or
EOQualifier thatHaveAPetDog = aPerson.PETS.anySatisfy(Pet.IS_DOG.isTrue());