Add methods to ERXKey that simplify creation of ERXExistsQualifiers. e.g., aPerson.PETS.atLeastOneSatisfies(Pet.IS_DOG.isTrue()); and aPerson.PETS.anySatisfy(Pet.IS_DOG.isTrue());
Change qualifier method name to be more consistent with existing qualifier names, add isEmpty and isNotEmpty relationship qualifiers also.
Add additional qualifier, improve Javadoc to make it more easily understood.
Merge pull request #553 from nekesto/ERXKey_ExistsQualifierAddition

Add methods to ERXKey that simplify creation of ERXExistsQualifiers.