Define a public Kernel.Pathname method

In mri pathname methods are added as private instance method and as public class method (singleton method) of Kernel.
Currently Kernel.Pathname throws an error because the private Kernel#Pathname method is tried to access.
make Kernel#Pathname public
added kernel spec file, which tests for the same behaviour as new spec
Merge pull request #3 from Benny1992/2.0

Define a public Kernel.Pathname method