adding support to list of Enums with Rule#has
refactoring on enum list support with has function
fixing method visibility and removing unnecessary interface
code formatting
refactoring for delegate the responsability to EnumFunction this way we dont need to change AssociationFunction implementation
removing unnecessary if
Merge pull request #28 from nykolaslima/has_function_with_enum

Supporting list of enums on associations: has(n).of(SomeEnum.class)