Fix valid parameter checks

lists/show and lists/subscribers can accept a list_id OR and slug. Both parameters should not exist for the method to throw an exception.

Probably needs fixing for other methods as well.
Merge pull request #47 from MauMaGau/patch-1

Fix valid parameter checks
add method for testing reducing repetition in twitter test
add test methods for lists
update param checks for getListMembers
update param checks for getList
update param checks for getListMembers
Merge pull request #48 from MauMaGau/list-tests

List tests